That great wyrm red dragon is already CR 26! If the CR boost is something like +6 then we are talking a CR 32 encounter, of course it is going to be big and deadly in lots of nasty ways!!
-Sorta-. The umbral template would change the nature of the beast by quite a lot. I'm working off the assumption that it's more or less the same as the umbral template from SS, mind you. If it is, the dragon is giving up a sizeable chunk of their natural abilities in exchange for the template abilities.
Gives up its massive natural armor bonus in exchange for a deflection bonus to ac equal to its charisma mod. That's a loss of about 30 ac, right there.
Ability to use magic items? Gone; it's incorporeal, so the only items it could activate are ghost-touch items.
No con score, and no change in hit die type. Dragons and undead are both d12 creatures - all of a sudden the colossal red dragon goes from being 40d12+400 (avg 660hp) to 40d12 (avg 260hp).
Can't snatch, can't grapple. Two of a dragon's mainstay abilities are taken out of the equation, right there. The umbral dragon can no longer play the "neutralize your friend and run away, what'cha gonna do now smart guy?" game.
Loses every special attack of the base creature. No breath weapon, no frightful presence, no spell-like abilities.
Loses the large suite of natural attacks a dragon normally gets in exchange for "an incorporeal touch attack". Attack, singular. As in one.