AA miles so much more useful than UA miles?
while AA usually has availability in flights to London, those are often on British Airways flights with huge fuel surcharges.
For other European destinations, AA miles are not super useful, though the addition of US has helped a bit.
For Australia, AA is terrible because of the restrictions that don't allow routing through Asia. Star Alliance has both United and Air Canada from N. America, and allows routing through Asia.
AA also does not allow stopovers at all.