I have a couple IOR Valdada fixed 36x42 scopes mounted on .223 Rem .308 Win F-TR rifles that I've used in 600 yd F-Class matches many times. The IOR Valdada Schott glass is possibly the best I have ever looked through, and these scopes work very well for F-Class, IMO. It is similar in specification to the Sightron, and has a floating dot reticle. I use the target scoring rings for hold-offs, so the lack of stadia/hashmarks isn't an issue for me. I have other Nightforce scopes with both floating dot and hashmark reticles. Both work equally well, IMO. However, I very much like an uncluttered reticle, particularly around the middle are, so I generally prefer floating dot reticles. Further, I rarely run above 32X magnification on the other NF scopes I have, even though some of them have top end magnification from 42X to 55X. My risk of cross-firing is too great when running at higher mag, and I also generally find that 32X is plenty of magnification for my eyes, even at 1000 yd.
The main [potential] drawback to using a fixed power scope in F-Class would be if the mirage becomes severe enough that the user would want to back the magnification down to clear up the image a bit. Obviously with a fixed power scopre, you can't do that, so if the mirage comes up really heavy, you;re stuck with 36X magnification. A second consideration with the IOR-Valdada is the 42 mm objective lens. It can be a little on the dark side, especially first thing in the morning or on very cloudy days. With its 45 mm objective, the Sightron may be a bit brighter, but I'd imagine it is still noticeably less bright than a scope with a 50-56 mm objective. Neither of these two potential downsides has yet become a deal-breaker for me with the Valdada scopes, but I do occasionally just have to live with a little mirage distortion or a somewhat dark image under low light conditions. If either of these issues might really bother you, I'd keep looking.