Thanks, all, for the feedback! I took a look at the Tripadvisor thread. Definitely lots to consider there. My initial take is that perhaps some Turo renters are not going into the arrangement with their eyes wide open - it seems like a lot of people are surprised about extra fees and policies that are actually quite well documented on Turo's site, and assume that it's just going to work like a regular car rental.
I think one of the biggest disadvantages with Turo is that if something goes wrong with your rental while you're out on the road and you need to make alternate arrangements, you still have to get your car back to its owner or arrange for the owner to come pick up their car, as opposed to a regular rental chain where there is likely an office nearby where you can have your vehicle swapped. This becomes more of a significant issue the further you drive from the Turo owner.
Something else I was thinking of in regard to Turo in England specifically is that from what I understand, England's MOT car inspection process is more stringent than most state car inspections over here in the US - so in theory that should give you more assurance over there that your vehicle is likely to meet basic safety standards than it would be with Turo here in the US, where some states don't even have a required inspection.
Another factor to consider is what we're used to driving here in the states - I drive a 22-year-old wagon, my wife drives a 19-year-old minivan, and I maintain both of them myself, so I'm not accustomed to the latest and greatest most comfortable automobiles and would probably be willing to put up with well-worn vehicles if the price is right.
It also seems that Turo has a lot in common with AirBnB in terms of how experiences vary from owner-to-owner and renter-to-renter. We've had a decent amount of experience renting with AirBnB, and quite frequently are willing to pay for slightly shabbier accommodations or more difficult to work with owners if they're in the area we want at a price we can afford. Obviously putting this amount of trust in a vehicle that you're depending on to keep you safe on the roads is different than renting an apartment with a worn carpet or a slow flushing toilet, but I guess in general it serves to illustrate our attitude on rental-by-owner experiences.
Still happy to hear any first-person experiences people have with Turo in England or other thoughts on this topic!