I would agree that some of the flying missions in GTA SA were pretty easy (e.g. using a jet pack or a VTOL jet), and most of them were pretty short (relatively speaking). But in terms of having to repeat the same mission multiple times, there were two missions that I found as difficult as anything in GTA V.
In one mission (“Freefall”) you have to fly towards an unseen enemy plane, do a quick U-turn, and then reach a spot immediately behind the other plane’s tail. The tricky part (for me) was that my plane was slower than the enemy plane (or at least not noticeably faster), so the timing was pretty finicky. After a number of failed attempts, I think I had to start the turn early and also be higher than the enemy plane so that I could build up some speed by diving.
In another mission (“N.O.E.”), you have to fly across the map while staying at a low altitude to avoid radar, fly through a target, then return back across the map within a time limit. It took me several times to figure out a path that wasn’t obstructed by trees or hills and that wasn’t over water (I had a hard time figuring out my altitude when flying above water).