Yes the issue with these L4D clones is that they have to improve upon the L4D formula, or have an interesting take on it. Vermintide and Payday succeeded on their own merits even though the core gameplay loop is very close (and it took each of them two games to get there).
I have tried both Back 4 Blood and the Anacrusis (in early access), and was pretty much over them after a couple of rounds.