You seem to be majoring on backhaul -- why? There's more to an ISP than just the backhaul between the telephone exchange and the core network.
In my mind not being a backhaul owner is generally an advantage. You say you're with Plusnet -- they'll exclusively use BT Wholesale for backhaul as they're part of BT Group. If BT's backhaul is congested then you'll be waiting for them to resolve that.
I've not seen a BT Wholesale complaint about congestion for quite a few years now.
The issue is Zen ARE a backhaul owner and they are growing and expanding their own backhaul.
They are migrating current customers off of BT and Talktalk backhaul on to the Zen backhaul.
The Zen backhaul is absolutely s*** particularly for anything 500Mb+
There's constant complains here and elsewhere over many months (maybe even a year now) from Zen users who have been moved from a perfectly functioning, full speed 24/7 backhaul on to the Zen backhaul.
Zens backhaul doesn't even seem to be congested. It's a configuration issue somewhere. Most of the complaints are of low throughput immediately upon the change of backhaul and it's all day everyday.
Zen seemed to make genuine attempts to fix this, even sending technical guys from their team to some customers homes.
That attempt to fix things has failed and the issue still exists to this day, with zero communication from Zen on it.
Their fix? Move customers back to BT Wholesale or let them leave. A few months later they migrate them back to Zen backhaul and the cycle starts again.
It might be area specific or a hardware specific issue that only affects a sub set of customers, so others might have a perfectly good connection over Zen backhaul. I don't know because Zen either hasn't gotten to the bottom of it or hasn't communicated it with customers. The only comment I ever seen (on here from Zen staff) was they thought it was an Openreach issue.
I wouldn't take my broadband for free from Zen now. I closed the last account I maintain with Zen around 2 years ago when 3 emails over 6 days went unanswered.
IMO Zen are a shadow of the company they used to be. I highly recommended them for many years but now warn against using them.