I'm on Zen FTTP and never had an issue with performance or latency. Can't say I've really had to use CS though!
As mentioned they have two backhauls, one is their own and they are migrating users to it. They migrated me and as a result my ping to London went from 6ms to over 20ms, and I struggled with consistent pings/speeds afterwards. CS attitude was '20ms is perfectly fine and your speed is above our maximum, go away'. I managed to get out of my contract as they broke Ts&Cs by changing my connection for the worse without informing me. I moved to Aquiss and all the issues magically went away. I suspect they've invested heavily in their own network and want to reduce their BTW usage hence the migrations.
With that said, some people have been migrated and their connection is perfectly fine, and Zen CS is a step above the likes of Vodafone/TalkTalk etc, so if you don't perceive any issue then don't go looking for any.