I can't speak for Neisseria specifically, but quite a few people didn't like some of the big changes in 8th:
~ steadfast, making large block infantry night unbreakable to anything but larger block infantry
~ "step up", not a rules so much as the concept that casualties in melee are pulled from the back, so models in the front of the unit can still strike when their initiative rolls around.
~ extra rank of fighting.
~ extra extra rank of fighting in horde formation
The rules above have a combined effect of really pushing much larger unit sizes than was typical in 7th edition, for both defensive and offensive infantry units, and discouraging a number of unit types, including: cavalry and chariots, which no longer break units on the charge thanks to steadfast and quickly get dragged down afterwords; elite, lightly defended units which previously relied on killing the enemy models touching them before they could be attacked back, things like witch elves or lahmian style vampire builds (unarmored melee relying on speed for defense).
~ putting skirmish units into a spread out ranked formation instead of just moving them in a 40k style cloud.
This is, in addition to being a significant downgrade for skirmishing units, just super awkward and annoying to play on the table, making skirmish heavy armies, especially skirmish heavy melee armies, just unfun to play with.
~ available dice dependent on a die roll rather than number of wizards
~ most dice you can throw at a spell set to six instead of limited to your caster level
~ caster level added to casting and dispel attempts
~ most powerful spells far more powerful than ever before
These rules combine to have some significant negative effects on the magic phase. For one, available dice don't scale with game size in any way, so magic can be far more overwhelming in small games, and arguably somewhat underwhelming in large ones. For another, if you're trying to cast spells normally - playing back and forth to bait out spells to get the ones you want through, then the caster level bonus overwhelmingly favors level fours, to the point that if you can't field a level four, your better off miscast fishing for super spells, and the fact that you can throw six dice and these powerful spells means far too many games come down to a single die roll.
Granted, 7th edition had its own flaws. Basic infantry just do not stand up to anything that hits hard, even when charged from the front. And basing magic dice available on number of wizard levels you're fielding saw some armies (tzeentch daemons, some undead) rolling 20ish casting die against other armies who could supply at best five or six dispel dice, and it was a big problem in itself that, imo, will need to be addressed.
Furthermore, for our faction in particular, the current rules and our current rulebook promote far, far greater varieties of list building. You can deliver blender lords in infantry or cavalry. You can run ghoul themes with horrors, ghouls, and ghoul kings. You can run elite infantry with double mortis and invocation support to try to win a game of attrition. You can run big zombie units with necromancers to bog down the enemy while spammed screamers tear them apart from outside of combat. You can run powerful death and shadow casters and just rip the opponent apart with direct offensive spells like purple sun and pit of shades. You can run fast aggressive armies built on fast monsters and flying monstrous infantry. You can spam ethereals and overwhelm the opponents ability to deal with them. There are lots of potentially viable approaches to this army, thanks to what is, overall, some very good internal balance for a GW product.
The same is not the case for our 7th edition rules. You can try different things, but you very, very quickly get funneled into the 'best' build - a big, wide unit of grave guard with great weapons. A bunker of skeletons behind them housing a vampire built for casting with the helmet that passes on his WS to said grave guard and the ability to raise ghouls above starting size. Flanked by multiple medium sized ghoul units that you can build up before combat is joined. Some necromancers in that bunker to up the casting dice pool.
The list more or less writes itself, and allows for very little deviation or experimentation. It was also stupid powerful - nigh unbeatable until daemons of chaos rolled around to break the game over its knee, and then dark elves showed up to burn the pieces that remained.
For vampire counts, balance - whether internal or external - just wasn't a thing in 7th edition, and that alone would make me hesitant to go back.
