If you upload a song at -12lufs, this is not a problem, if you upload a song at -16lufs, then this can be minor issue. Basing the target at -14lufs
I'm compressing around 3-6dbs on most tracks to make balancing easier and my LUFS normally hits -8 (loud) to -9(competetive), which sounds great to me, there is room for percussive transients to hit more than hard enough.
The site doesn't compress to get you back to -14lufs, if you upload at -12lufs, it just turns it down, you would never know unless your song got played back to back to a song that was a consistent -14.
I don't have much knowledge if sites would compress an overly dynamic song to bring it up to -14 though, i doubt it.