The only concern you should have is that you'll want to contact United and the operating carrier every week to ensure that the itinerary has not been canceled. And you'll need to ramp that up to "once a day" in the week before travel.
This is mostly good practice anyway — you really ought to reconfirm reservations about once a week to check for schedule changes, seat changes, or equipment swaps. The part which is a little dicey is the recent reports of UA cancelling third-party award bookings with short notice (within 24 hrs in the case of a CM award reported recently).
But as long as you're willing to not only (1) book the trip for someone but also (2) babysit the reservation for them and act as a travel agent in watching out for them, it's fine to book an award for friends. If you just do (1) and not (2) it's kind of like giving someone a slightly unhealthy animal as a gift — uh, gee, thanks, but now I gotta take care of this forever? I guess I'm happy but, uh, jeez.