As a counterpoint
I've been on Zen's FTTP for a couple of years now without any problem.
About a month ago, the connection dropped, and looking at my router logs it was full of PPP timeouts. Called Zen, went through troubleshooting with someone who knew what they were actually talking about, and they determined an Openreach fault.
First Openreach droid turned up, looked at my ONT and declared that because the PON light on my ONT was a steady green then there was nothing wrong with the Openreach side of things and refused to investigate any further. He logged a response back to Zen saying that the fault was with my equipment.
I spoke to Zen, and they escalated within Openreach and a second chap turned up a few days later, ran some tests and declared there to be no issue at all between my house and the exchange, and disappeared.
I called Zen again and explained the quandary of my Schrödinger's internet connection, that was apparently simultaneously fault-free and non-functional, and they escalated once more within Openreach.
A couple of days later, I had a call from Zen to say that Openreach had discovered a fault in one of the network switches connecting the FTTP head node to the backhaul connection, and the switch was tagged to be replaced in a couple more days.
Sure enough, the connection came back a couple of days later, and Zen gave me around £75 as compensation for the lost service.
Now, I don't know how my experience would have differed with other ISPs, but having had dealings with Virgin Media and Plusnet in the past, I have doubts as to whether I'd even have been able to talk to someone sensible, let alone get it resolved.