Hi Bob,
For the sake of discussion, we'll call Heli-Coils & Recoils the same thing, so it's them versus Time-Serts. Those being the only types I have at least some exposure to (I know there are wood-workers types, but that's completely foreign to me).
Now I've never used a Time-Sert, and it wasn't until recently that I saw a picture of one of their kits, showing the included counterbore. It makes sense, but I'd previously envisioned it requiring a milling machine setup; but it still seems like a lot of extra work.
I've got a pretty good assortment of Heli-Coils, from 6-32 through 3/8, both fine and coarse pitch, including a LH set for 3/8-16, plus M6 to M14, along with the 1/8-28 BSP to suit the rocker-arm oil-feed (a size that I did not see on the Time-Sert website...).
The point I want to make is, while sometimes a Heli-Coil repair seems like a gamble__that tiny 6-32 to save a Tillotson carburetor comes to mind__I have never had an application that did not work. A couple of times, I suppose I screwed up the installation and had to pull out the insert by unwinding it, those things can be really long when you straighten one out btw, but I've never had a repaired thread fail once it was put back in service. I'm real happy with that.
The next time I need to repair a thread in an aluminum head (preferably a McCulloch and not a BMW) I may give the Time-Sert a try, but their kits, no doubt due to the extra tool/step required, are noticeably more expensive than the Heli-Coil/Recoil brands.