Usual flawed and partial statistics
One thing is for sure: you pay peanuts--. Having been inside the industry (not any more) for some time, the main problem is the race to the bottom on price. That means less money to run the network, less to pay good support people, etc.
Zen and AA have persistent good reputations going back years. I'd add Eclipse, who I've been with since 2002, and despite them being acquired by Kcom, they also provide good service and good support.
This report also fails to explain to customers who they are actually dealing with. For instance, Plusnet is owned by BT, and is only partly independent, most particularly at the network level. Post Office is entirely operated by TalkTalk - and so on.
And below that of course in all but one case you are reliant on the Openreach local loop to the street cab (for VDSL) or the exchange. If you report a fault, your ISP has to call OR. Now OR's field staff seem very good (afaik they are actually employees, not contractors), but there is a chain of command problem there.
The "one case" is of course VM. afaik field staff are contractors, although I think that may have changed. Problem is that DOCSIS, being a shared medium on the "local loop" can suffer congestion on the last mile, and also has inherently more difficult latency characteristics even when not saturated. Add to that the problem of street cabinet maintenance: cable systems are rather like token ring, and any single loose screw-on F connector can mess up the whole segment. xDSL doesn't have these problems - if your Krone connection is bad it only affects you - but does have the well-known speed/range problem.
Call centre staff have no clue about all of this, and no control even if they did.
Marketing is very guilty: "Fibre broadband" (no it isn't), "Speed up to xxx" (downhill with a following wind if you live next to the exchange and the entire route to the destination (let's say Facebook) is uncongested). "Lowest price" (no, we can't afford to pay to keep it working or answer your calls). Etc.
It's a wild west industry still.