Personally I have no reservations using them where the bolt goes in and stays there, but a better solution to helicoil in a temp and vibration environment is what is called a "Timesert":
We used them for years to repair stripped plugs, some the result of automakers posting their 100K before the first tuneup. Ever try and pull a plug in an aluminum head that long?
p.s. I've seen too many helicoil inserts come out with the plug, or worse, thread inwards with the new plug to fall into the cylinder.