Hi Charles,
We've been using UPX --force --ultra-brute (the deepest compression among its 70+ intrinsic algos) for at least 15 recent years to compress the FBSL binaries (Fbsl.exe and Fbsl.dll) v2 and 3+, as well as most end user executables written in FBSL containing BASIC and/or Asm and/or C code, without a single fault attributable to UPX as reported either by the devs or our end users.
To the best of my knowledge, Eros has been using UPX as well to compress his binaries for at least as long as we did it and ever since I became aware of thinBasic, and to the exact same effect.
So, in my opinion UPX is faultless and very, very fast even in its deepest compression variations. Modern AV software is aware of UPX intrinsics and signatures, and thus it does not throw false alarms just on account of the executable having been exe-packed the way it usually does when dealing with other less popular and more cryptic executable packers, especially those that break the PE format. Just make sure to use the keys that leave the exe/dll resource section(s) with the manifest and version info resources uncompressed for the AVs to be able to detect their availability and consistence with the Windows PE format guidelines.
