I almost always transfer my UR to United to book flights on the United site using miles, but I just recently purchased our flights for a Peru trip this summer from the Chase travel portal using UR directly. I was not finding any saver awards on United, so four roundtrip coach flights would have been 400,000 miles. I played chicken for a while to see if saver awards would pop up, but there were a couple of specific flights that were far preferable for us from an elapsed time, layover and arrival/departure time standpoint, so I ended up pulling the trigger on the Chase portal when these same flights were bookable for just slightly over 90,000 points per roundtrip, so just over 360,000 for all of us. I know I will be kicking myself if saver awards do pop up on United, but my solution to that is just not to check, lol.
ETA: And there were no taxes owing with the booking. Just the UR.