I think it can vary quite a lot. Remember that all the networks were originally built by different companies to different standards. Most have been through several different owners since then, each of which had their own different standards for maintaining the network. Even now I believe that some work is outsourced to different companies in different parts of the country.
Where I am the network is fairly good, mostly because it was fully rebuilt a number of years ago, so is newer than most. I often read about people with problems where the cabinets are falling apart or the network is oversubscribed, but have never experienced anything like that myself. I have seen issues whilst visiting family elsewhere in the country, so it does appear to vary depending on where you are.
I guess there are two main issues here :
1. Physical connectivity integrity (i.e. whether your RF connection to and from Virgin Media is secure and someone can't unscrew your connection or cut your cable)
2. Network connectivity (i.e. whether Virgin Media's network configuration and infrastructure provides the internet connectivity that people expect based on their local connection speeds)
There have been constant rumblings that Virgin don't engineer some aspects of their network particularly well - and run it 'hot' (so lose packets - in the expectation that lost packets will be re-requested and re-sent)
This ignores the quality, or lack thereof, of their Superhub in router and WiFi access point terms (Virgin's hubs have been hit by an Intel bug on their main SoC, which I believe is less of/not an issue if you run your Superhub as a modem only)
Anyone who cares about decent network connectivity is likely to have ditched the router and WiFi access point portions of their ISP router - and ditched their modem if they are allowed to. (We binned our BT HomeHub ages ago - replacing it with a Draytek Vigor VDSL modem and a Ubiquiti EdgeRouter-X and Ubiquiti WiFi Access Point. The Draytek is BT approved too..)