...Nothing is wrong with niin, humans, deigan, Nightwalkers, Melenians, xuhrl-njok, tarken (you do not capitalize species except for Melenians and Nightwalkers, since those are from place names (Melenians from Melenaoth/Melenian Islands and Nightwalkers from Neitwakar) ... it is one of my greatest literary peeves, the unnecessary capitalization, and actually makes me physically cringe whenever I see it). 

Aemoten, while immortal, is not overpowered, nor more powerful than Jaelnec or Thaler or Olan (he is making Thaler stay out of the current fight because of her mental condition, not because of her skill - if Jaelnec was suicidally depressed, he would not allow him to fight, either). 
I actually estimate that Jaelnec would have better chances of winning than Aemoten in an one-or-one swordfight (or at the very least, I have tried to write him as being slightly inferior to Jaelnec in that regard). Aemoten has nearly died several times, too, and for the matter has been seriously injured notably more often than Jaelnec. He can request aid from the Six-Eyed God, true, but this is the one god who always takes something tangible in turn for favors. I am counting down on what he gives, and I tell you, Aemoten is quite strongly in the minus at the time being. (In a sense, an actual Favored One of any other deity is more overpowered, since although what they can do is more limited, they can do it indefinitely without a notable cost.)
The resurrections aren't exactly as easy as Thaler imagines it, either, the same as losing someone dear to him has never gotten any easier. (Do you also insist that if a mother loses one child, losing the other would not be as painful anymore? No! That is ludicrous!)

...I usually play average characters. Not weak. Nothing particularly spectacular, either. Not strong enough to effortlessly defeat anything but the lowliest grunts, but not so weak as to be violently butchered in the very first fight. 

And I don't see where Legion says his people are harder to defeat than humans? I'd say they are about equal to humans in that regard, and far more easy to defeat than, say, tarken or xuhrl-njok, especially when not in numbers. The del-korm are more physically powerful, but put one against a tarke or a lohk, or a deigan or xuhrl-njok using magic, or just a bunch of basic commoner humans with crossbows who are in a fortified position, and the physical superiority disappears.
I know we had a bit of a run-in with power-scaling back in the day, but do not leap to conclusions prematurely. It does not do anyone justice. 

They aren't playing their own races because their own races are more powerful. These races aren't more powerful than the established beings of Reniam, just different. They are playing them because of variety, and because they like those races and would like to try them in a different setting.