I had an absolute nightmare with Sky Fibre and downtime (Speeds were really good though)
Virgin I had from 2mbit to 152mbit over the years and had downtime hours/day but nothing as serious as the trouble I had trying to get Sky Fibre in. Some things just weren't meant to be
I don't think I was ever throttled on Virgin either although I think where I lived played a big part in that, so much that I had an entire cabinet to myself. Was never throttled on Sky either though.
So it works both ways to be fair. Both products do what they're meant to do for the majority. Bound to get complaints for either.