AA is not as far along in its merger with US as DL with NW (long completed) and UA with CO (much more completed). As such, currently, for another year or two, AA planes are a little less "predictable" than might be the case at DL or UA. Though non-merger changes (like DL recently did with C+, and is it done with that finally yet or not>) can happen at any time and possibly be disruptive at least to plane layout predictability (while half the fleet is old style and the other half is new style).
The plane layout not being predictable (if you book in advance) can mean that the seat you reserved is not the seat you get (much more likely than would be the case if plane layouts were consistent). So it can impact quality of the travel experience if you care what you seat you sit in.