If I understand correctly VM provide two services, if you're not in their one of their cabled areas you get the same poorer connection type as the rest of UK, including ZEN so at best half my current broadband speed. I assume VM cable means they have better control of the overall service as they own the infrastructure between them and the customer whereas anyone else's "superfast fibre" is the re-badged BT service.
The picture for VM might look better if it was judged on their proprietary cable only.
I've been with VM since they cabled our street - over 20 years. I appreciate the fast broadband and it usually delivers the quoted speed (150Mbps, option of switching to 350Mbps - but £££), none of these "up to" speeds everyone else offers. I've got no gripes with support, outages are rare and usually fixed promptly. My issue is the continual price-hikes and I seem to be trapped and paying £800 a year (TV, Phone, Broadband). To switch I'd tackle the issue by getting a second broadband service alongside VM 'till it's proven good enough but that means getting a land-line installed £££ and then replacing the Cable TV (broadcast TV reception is patchy here) - but then if the alternative proved to be poor, I'd probably be on a minimum contract length so dropping either that or VM would be a difficult choice.