Lower int makes them slower to learn in general.
Added to that lower str to be able to carry less
spare weapon (to hit vulns etc) and maybe it
is just me but felars seem to "accidently" step
out of shadows even when everything is perfected
(from my lowbie felar thief experiences in recent past)
Add in being a rager, so no potions/pills to aid in
maximizing chances of success. But take all that with
a grain of salt as I have not played an assassin to
assassinate since they changed however long ago
that was.

All in all, I think felars are more suited to be a combat
assassin as opposed to an assassinate assassin.
Especially as a rager, but I think that applies for most
every race.