The Early Game of Sevtech is a very slow start, gathering resources while figuring out what the vague advancements are telling you to do, and it only gets worse when you have to sit around for literal hours running around a darklands biome waiting for something good to spawn, like currently I'm trying to find Abyssalcraft zombies so i can get the green flesh. I have found absolutely none, meanwhile my partner found 3, in the hour or so we've been searching. On top of that, mob hunting is a pain anyways, as even in the hunting dimension, you'll see very few mobs. Please raise the limit of hostile mobs or at least fix some spawn rates, because this just makes the pack grindier than it needs to be.
I've found its extremely easier to find them after a restart, when other mobs are deleted from the world and thus the cap is no longer at max. Yesterday after I posted this the server restarted, and I found more abyssalcraft monsters that I'd ever seen before, just make sure to be in a darklands when server restarts, although we shouldn't have to.
We've adjusted the cap several times, and even removed the fish swimming around as these counted against the cap and caused even less mobs to spawn, there is a vanilla limit for mobs and we do hit it on occasion, this would explain why you see them shortly after the restart before the passive mobs are loading up and eating away at the amount of mobs in the world. There's also some weird interaction between sponge and abyssalcraft monsters not spawning nearly as well as the do in single player. It was reported to author who basically shrugged it off. My solution was always to go looking around for dungeons, there are several types of buildings with spawners in them that have the zombies, and the shadow creatures are also available that way, it just tends to be less obvious as they follow a more traditional vanilla dungeon rather than the huge buildings zombies are held in.
Im pretty sure there are abyssal zombie spawners in the over world and farm world. Basically yeah what wynd said
There are oodles of them! Here's one in the farmworld for you. 2892 37 -2547 If you are staged high enough you'll have no issues tracking them down.