Generally, MVNO customers have lowest priority on a network. I know AT&T, for example, gives network priority to Postpaid first, then Prepaid, then MVNOs. So on a congested network, you'd be the first to be kicked off.
Generally speaking, I don't think the experience would be as bad as you say. Yes they get the lowest priority but given how good AT&T's network is, you generally won't be kicked off and would probably incur a reduction in speed. The reality is, you aren't going to need (insert super fast speed here) on a day to day basis. There are exceptions of course, such as tethering or streaming content, but really, how often would you be doing that and need it? This is why I don't buy the 4G argument, especially when there're carriers that ask us to pay more simply to be on 4G.
I think that's going to depend heavily on where you are. I was in Chicago last week for work and found out the hard way that VZW LTE is severely over capacity inside the Loop. I'm a postpaid customer and had to force my phone to CDMA data on a few occasions just to get a usable connection.
Knowing that I'm going to be travelling a fair amount in the near future on business, I'm not sure if it'd be worth gambling that the host network for my MVNO wouldn't be similarly over capacity elsewhere resulting in me getting lower priority.