Yes, it is safe - but don’t take my word for it - go to their support forums and even without looking myself, you will not find any of the zillion people participating in many many years ever claiming that FSUIPC stole their candy.
OH ONE THING: Like any other software, make sure you download it directly from their website. FSUIPC is often re-posted, re-hostef download elsewhere; i would not touch those with a 10m stick.