Most of the comments I've seen being unhappy with Zen are because they have seen their connection have low latency using their Manchester PoP but then another time end up going via London. Its less than optimal (as there is no way for the session to intelligently choose the shortest route, its purely random) but somewhat a none issue when you consider most ISPs only go via London, so you'd never see such a discrepancy. Its basically that for some people, sometimes Zen is more efficient than most ISPs so people notice the difference.
They have had single thread speed issues in the past, but then what ISP hasn't?
Zen are just a more complicated ISP as their backhaul can be Openreach, Talk Talk Business or Zen, depending on what provider is available at your exchange. Their future plans are very ambitious, to have metro rings in as many cities as possible so backhaul can be routed more efficiently and presumably so they can sell plain ethernet packages directly to multi-dwelling buildings.
From my own experience, there's really not much between Plusnet and Zen (Talk Talk Business backhaul). Both can differ significantly in first-hop latency depending on a given PPP session, but tend to work out pretty much equally in latency once out on the Internet. I do see ICMP packet loss on Plusnet frequently, but its hard to say if its always real-world loss (it sometimes is as I had to move my VPNs to Zen due to it causing dropped connections) or even if its just because my Plusnet line has far more errors than my Zen line.