Back4blood doesn’t suck, but it did really take a while for the game to be tuned at the harder difficulties. It’s a solid upgrade over l4d2 in every way, but doesn’t benefit from nostalgia goggles. I wish it had l4d vs mode and mod support, but I still put in a hundred hours or so (haven’t checked dlc) which is pretty reasonable value.
The problem with back4blood is that it’s good: not great.