caveats in rail-to-rail I/O opamps?
The only thing I think is constant across almost all rail-to-rail op-amps is they can source or sink only very small amounts of currents near the rails.

E.g. a RR OA may be able to swing \$Vcc-0.1\$ to \$Vss+0.1\$, but only with a \$100K\Omega\$ load on the output. With a \$1K\Omega\$ load it may only swing to \$Vcc-0.5\$ to \$Vss+0.5\$.

I haven't seen very many RR OAs that this is not true for.