I've had Grundy for seemingly forever. No change in rates since I got it. Agreed value policies. Only thing I don't like about the whole thing is that my parts stash isn't covered. Normally it would be, but South Carolina doesn't allow for it. Bastages.
I just snapped a few pictures all around the cars, and sent them in and got my quotes and all that. Pretty reasonable when you think about it. No mileage constraints, although it needs to be in a locked garage and all that rot and not your daily driver.
I'm hoping to never personally find out how they pay out on a loss, but a couple people I know had some incidents and they said they got the estimates, approvals, and they cut them a check. No problems.
I'm guessing your bigger collector car companies do it right for the most part, so if you go Hagerty, Grundy, ACI, whichever, I think you'll be ok. Some of the regular insurance companies have jumped into the collector car insurance market. Allstate, State Farm, and even Geico has a collector car insurance, although I'm not sure about them in general. I'd stick with the people who's business model is founded on collector car insurance. JMO.