The downsides of Dreams of Love and Happiness Gu were as follows:
For one, it required a bead of Grape Green Immortal Essence to activate. While this was to be expected from an Immortal Gu, it was also quite a steep price. Fang Yuan had methods to strengthen his soul now. In many cases it was just easier to try the Dream Realm over and over again and wait for his soul to heal, even if using less than ideal methods than Guts Gu, rather than wasting a bead of Grape Green Immortal Essence. Immortal Essence was a highly treasured asset and could not be squandered wastefully. Each use had to be carefully weighed against the potential benefits.
Secondly, it wasn't as if changing the contents of a dream realm did not come without cost. The attainment that one could gain from exploring said dream realm would also be lost to a certain extent. The reason being was that Dreams of Love and Happiness Gu also had some Emotion Path foundation, and sublimated most of the remaining attainment to attainment in the Emotion Path. If one was only trying to escape a dream realm - that was fine. However, if one wanted a specific path's enlightenment, it was naturally a waste.
Before Fang Yuan had no attainment in the Emotion Path, he would still end up gaining something from the dream realm in question even if he used it. The issue was that he was now a Great Grandmaster and could not raise it any further from dream realms.
As a matter of fact, most of the attainment from his dream path tribulation had been converted into Emotion Path attainment, which is why he had directly jumped from being ordinary to a Great Grandmaster in it.
Since he could advance no further though, it was truly a 'waste' of a dream realm's attainment to use it now.
Third, and its biggest drawback - was that it was not without its own risk. A lower risk than otherwise exploring a dream realm perhaps, but a risk nonetheless.
Dreams of Love and Happiness Gu converted dream realms into pleasant dream realms, but this carried a risk as well.
If one became too invested into the pleasant dream scenario, one may wish to never wake up. A Gu Master had to endure happiness and contentment until it reappeared in their dreams, signifying that they could finally leave.
If they did not, the Gu Master in question would eventually be trapped within that dream realm forever, never emerging from it.
For many, happiness was more difficult to endure than difficulties. Many Gu Masters could endure calamities and tribulations. But how many could let go of a dream when everything they wanted was in front of them - as illusory as it might seem?
If one was too content within the dream realm, they may foolishly choose to willingly never wake up. This did not work on Fang Yuan, who had broken through the illusion in an instant, but there were many Gu Masters who would prefer the pleasant dreams that this gu worm would grant them compared to the harsh realities of the world.
Too much of a good thing could be bad.
Humans were not meant to always be content and satisfied all the time.
And moreover, the 'joy' granted by Dreams of Love and Happiness Gu could not be said to be true happiness. It was illusory satisfaction, much like how a rainbow was nothing more than light being refracted through raindrops. Anyone who tried to 'catch' a rainbow would end up with nothing.
In the same vein, discontent was not a bad thing. Humans needed discontent in order to advance themselves. Someone who was comfortable where they are would not seek to better themselves, or to seek out the truth as much as someone who was unsatisfied.
Birds would never learn to fly if they did not first leave their nests.
If Siddhartha had been completely content living in the lap of luxury within his palace, having attained everything a mortal man could ask for - would he have abandoned all of this and set out into the world? He had seen the truth that family, fortune, money, and security - they were all but temporary as he set out on a journey to discover what was eternal. Would he have finally found true enlightenment and happiness were it not for his discontent with the material realm?
No matter how pleasant the dream was, one would have to eventually wake up to reality! If anything, the sweeter the dream, the more bitter that awakening to reality would be like. The sweeter the dream, the more one might want to deceive oneself that they were not in a dream, but in reality instead.
If Fang Yuan, during his first life, had indeed awakened an A-grade aptitude and was able to achieve all that he had wanted within Gu Yue Village, would any of the adventures that followed had happened? He would have, like Siddhartha would have had he not been discontent, stayed within the confines of Gu Yue Village - leaving the vast wide world unexplored. Would he have stumbled, much like Siddharth did, upon the truth of the world like he eventually did - that there was nothing worth pursuing other than Eternal Life if he had not left the comfort of his home?
Fang Yuan considered himself to be above such a thing now, but dream realms had a way of amplifying emotions.
The best way to deal with this was to allow it to cool down after use - that was the best cure when one's emotions were running amuck. Either that, or Fang Yuan needed to devise Emotion Path methods to deal with this side effect.
This could be seen as a slight silver lining in the matter that Dreams of Love and Happiness Gu changed attainment to the Emotion Path - it built the foundation for countering its greatest negative effect as well. Additionally, this attainment could also help counteract the emotional lability encountered while exploring other dream realms.
Fang Yuan's mind was already racing with several ideas on how he might counteract dream realms from affecting his emotional state too much. Great Grandmaster was a high level of attainment - at this point, a Gu Master could use that path to mimic the effects of other paths.
Fang Yuan simply needed to use Emotion Path to mimic that of the Dream Path. Since Dream Path was still in its infancy, this was easier said than done.
However, Fang Yuan had the advantage of his future knowledge - there was a killer move known as Unravel Dream that was quite useful in solving dream realms.
The issue was that its core gu, Unravel Mystery Gu, was not in his possession.
But, Fang Yuan was now also a Wisdom Path Master - a side effect of becoming an Emotion Path Great Grandmaster and also having plotted and schemed for a good portion of his life. Emotion Path was a branch of the Wisdom Path, which is why he had been able to build upon an already solid foundation to get to this level, but could get no further without directly exploring a Wisdom Path dream realm. Emotions were just one part of the Wisdom Path - thoughts and wills made up the rest.
Still, Unravel Dream was based on a Wisdom Path killer move. It was the Wisdom Path mimicking the Dream Path - made by a Great Grandmaster in the Wisdom Path, but Fang Yuan lacked the kind of insight in order to fully change the core of the killer move.
He had a Dream Path Immortal Gu now, as well as somewhat reasonable attainment in the Wisdom Path.
Theoretically, it should be possible - for that matter, even easy, given that he was using a Dream Path Immortal Gu directly as the core and not a Wisdom Path Immortal Gu.
Much like with trying to turn the Bone Chariot into Blood Chariot, the puzzle pieces were there - he felt that he could reach out to them, but there were enough missing that he couldn't fix it.
It would take a good number of years and some trial and error, but he was sure that he could change Unravel Dream and adapt it to accommodate Dreams of Love and Happiness Gu eventually.
As it was though, it would be time before dream realms fully manifested themselves.
As for its feeding, Dreams of Love and Happiness Gu fed upon the joyous dreams of people. It was easy enough to feed for someone like Fang Yuan who had not only Gu Yue Village to rely upon, but eventually would have a massive human population in his aperture. Sure, it caused some of the villagers discomfort at having their dreams eaten away, but that was hardly something that Fang Yuan would lose any sleep over.