A friend of mine purchased one and together, and separately, we tried to figure out how to use it accurately, but were never successful. We were able to find pregnancies, but we also found some pregnant stallions, so we knew that wasn't right. We weren't comfortable that we could trust our results, so just continued to use the vet to do internal ultrasounds for us. She has been pretty much 100% accurate starting at about 14 days gestation.
The machine is safe though it is external, so no chance of hurting them.
Although they are ultrasound machines, they don't show you a 'picture' of the foal. The version that my friend bought has a graph line with markings along it that represented different 'depths'. The machine sends out ultrasound waves and records the 'depth' that it detected an echo. That detection shows as a spike along the graph line. A positive result for pregnancy is a spike at a particular 'depth', (I want to say at 13???, but its been a while). You will also get spikes at other depths, but that might be the bladder or some other thing in there that creates an echo.
Knowing exactly where to place and aim the ultrasound is the key. We tried and tried but could not get consistent results. She eventually sold the machine.