Harm is definately a good example, destruction is a bit weaker but we can still go with it
But still, there are all sorts of damaging reflex save powers that evasion works against, and only a handful for fort and will combined.
I'll go through, as I have a few minutes
Blight (only if you are a plant though)
Chaos Hammer (only if you are lawful)
Cloudkill (this one is actually kindof cool.. instead of the poison dealing half of a d4 con damage on a save you take none)
Cure spells (if you are undead)
Destruction (for a 7th level, single target spell, 10d6 is mean, but no where near as bad as a few alternatives..

)
Disintegrate (avoid the 5d6 damage from this 6/7 level spell)
Finger of Death (avoid 3d6 + level 7/8 level spell)
Harm (only if you are living)
Holy Smite (only if you are evil)
Horrid Wilting (I like!

)
Inflict spells (only if you are living)
Order’s Wrath (only if you are chaotic)
Phantasmal Killer (3d6 damage, if you fail the will save and then make the fort save)
Prismatic Spray (if you roll a 4, saves from d6 con damage)
Prismatic Wall (from green part, saves from d6 con damage. Of course if you are making this save you are probably dead anyway)
Shatter (if you are a crystalline creature)
Shout (way to avoid half of 5d6

)
Shout, Greater (negates half of 10d6)
Slay Living (save yourself from 3d6 + 1/level)
Unholy Blight (only if you are good)
and that is it. Reflex saves are easily 4x as common as will and fort combined (for spells that it will work on).
Mainly, if you are not an undead, crystalline, plant mettle doesnt really do a whole lot for you unfortunately.
I like it because it is an interesting option, but I hardly think it is overpowered

If the evasion one is not overpowered, then the mettle one is roughly 1/4 as powerful.