I dunno... the mirror walk and reflective SR are potent, but not enough to make them significantly more powerful. Mirror walking is good for out of combat movement, but as a DM tool, they can move in any number of ways to confound PCs. As PCs, they can't take anyone along with them (I don't believe), so their travel ability is marginalized.
The SR is nice, but their HD is rather low, and they attack power is nothing to write home about. A canny opponent will just beat the crap out of a Nerra in melee, or summon something to fight. They aren't at the top of their CR's melee combat ability, so a beefy summoned animal will probably do the job.
As PCs, the LA is enormous. With LA of +11 for the most powerful, you'll have SR of 19 at level 18. That is so low that an equal level opponent is guaranteed to land all his spells on you. The mirror walking ability is no more powerful than a Shadow Walk spell. The Reflective SR is by far the most powerful ability of the race, but it still is only worth so many levels... 3 at the most.
Acid Testing against a Flesh Golem, the golem has a host of immunities, way stronger ability scores, significant natural armor, DR, and complete magic immunity at a much lower LA.
I'm not blaming anyone here, of course. I just wanted to make sure that I wasn't missing some major ability that the Nerra have that would be worth a lot of levels.