Define "viable".
A Monk 20 can be pretty effective against CR Appropriate Monsters, especially given an intelligently selected set of magic items. But a [insert Class X / Class Y / Class N / Class O, where X+Y+N+O = 20] can do it better (lets not even mention that a Wizard 9+ can probably outshine that same Monk 20).
Monk 20 is "viable" (whatever that means in this context), but Monk 9 / Shiba Protector 1 / Drunk Monk 10 is perhaps more enjoyable. Perhaps not.
"viable", among other things, means that out of combat the character has things to do other than sit in the corner while others solve all the problems.
Also, if a fully optimized & fully prepared level-20 monk doesn't have a prayer of surviving (1-on-1) an encounter your average titan on a bad day, then I guess it's not really viable. Even the Scout is better as a striker - and it has skills & blindsense on its side.