I have a pair of Festool and a pair of Axminster threaded rail clamps. Both are excellent quality, and I couldn't choose between them, apart from the price (Axminster ones are slightly cheaper).
The Festool ones live with the track saw, so get more use. I have had them loosen up, but it's unusual. I suspect that's because I try not to overtighten them (and thus don't do them up enough sometimes). There are two issues with overtightening: Firstly you bow the track, so the cut can be off-square (near the clamp), and if you clamp hard enough you risk dishing the track permanently.
To be fair I'm using Makita rails - the Festool ones might be thicker or better supported. But I have permanently damaged one piece of track (somehow - can't remember doing it), which is quite annoying.
That said, in an MFT environment as hold-downs for the work, either would be excellent (that's what I bought the Axminster ones for). And yes, both Festool and Axminster do fit Makita rails.