I can't speak to a silver mountain because I've not used one, but I can't think of a single meaningful downside to the ShotMarker.
The biggest downside I've seen is the minimum velocity of 1250ish FPS.
There are some decent F-TR loads that don't make 1250 FPS at 1000 yards. It is suboptimal to have to work up hotter loads just to get the target to work.
Not sure if Silver Mountain has the same drawback or not.
At some point, the NRA may have to weigh in. As the current High Power rules on electronic targets only require shooters to ensure their bullets are not subsonic when they reach the target. How long is the NRA going to keep sanctioning matches where the targets may not record all the bullets reaching the target meeting the NRA rules but not the target requirements?
9.27.2 Willful Use of Subsonic Ammunition on Electronic Targets – When electronic target systems are used, no competitor shall knowingly use ammunition which is subsonic at the time it impacts the target or reaches the target line. Subsonic rounds may cause damage to target systems and may result in erroneous readings.
Any microphone based target has the limitation of the ammunition being supersonic at the target, because the microphones are detecting the timing of the shock wave coming off the bullet. You sort of have to work within that limit if you want to use them.
Just curious, but what good F Class loads are you seeing trouble with? 6mm or .224? Most of what I see guys shooting is still around 1300 at 1000.1000 yard shooters have been solving the supersonic problem for a long time for accuracy and stability issues. But let's be honest, the bar for some of these electronic targets is higher than supersonic (1080 fps) at closer to 1250 fps. Lots of good high power and F-Class loads that are supersonic at the target (>1080 fps) don't register on ShotMarker.
It's a limitation that both private users and match users should be aware of. Simply saying, "Bullets need to be supersonic" at the target to register is misleading. The details are important. Some systems may register bullets down to 1100 fps. ShotMarker is not one of them.