It's great to see the mod is still being worked on, hopefully the feedback can help with that.
I come from Project Ozone 3, and AbyssalCraft isn't the focus, it's one of many mods. With that out of the way...

TL;DR; The grind is way too much, it's the only mod I'm considering cheating through

These are the major overarching reasons:

Not enough interesting milestones.

The progression system lacks some "kick". Progression in AC is often hard-gating subsequent progression. Compared with say EnderIO, with each successive ingredient you manage to get you feel a significant power up. This both gives a sense of achievement and gates progression in a softer way.

Insufficient automation

Rituals are fun... for the first 5 times. At some point it's just repetitive mechanical motion, and that's tedious. Other mods usually solve this kind of repetition by introducing automation for earlier content after reaching later content. The automation itself is also great content, it is puzzle solving and optimizing at the same time.

Missing adventures

The dimensions have great potential to become adventures. But right now they're rather bland and doesn't provide incentives to explore them. This has been mentioned multiple times in other issues. Having some place to adventure can be a goal to aspire to, bosses doesn't have to be the only end goal.

It might be worth pointing out, and I'll be blunt here, that it's better to have a short amazing experience then have a long, grindy and dull experience. To that extent, personally I'd consider the dimensions being more suitable as biomes.