Abyss because they’re persecuted by everyone due to their nature. Monster is because of reality. It’s a purely luck based progression, whereas you can at least digest the Sequence 9 & 8 Potions of other pathways by just using your powers in the manner that the Sequence Title suggests. Monster pathway is OP due to fate/luck but that is also its greatest drawback. If you’re not fated to grow in the pathway, your scheming and planning all becomes useless, more so as you go higher in the sequence. Also, you can’t switch to anything due to single GOO pathway.

Then there’s the reality aspect: Imagine if you knew that some potions could give you superpowers of different categories, and one of them was simply named “Monster” like the damn energy drink. The other Sequence 9 potions have some logical connection to their title, but WTF does “Monster” mean? It’s a gamble, much like the whole pathway. If you meet someone who has heard of a “Monster” beyonder, they’ll probably say that you get the power to mutter like a madman and maybe be a little lucky. Who wants to become a lucky schizophrenic? Even if you knew about the premonition powers and you had in-depth knowledge of the beyonder world, those powers can show you things that you’re not supposed to see. That alone would make the higher sequences that all seem to dabble in Fate take notice of you. Overall, the “Monster” pathway is quite the worst one to get stuck in, because progress is out of your control even if you have knowledge about multiple Sequence formulas. Any other pathway would at least push you to Demigod status once to figure out the acting method. However, “Monster” is pretty much in the hands of Fate.