I’m actually leveling a Forcewave Tactician currently, self found, and so far it’s been completely fine. Only just about to hit Elite, though, so I can keep you posted. It will work OK with greens, you just won’t be able to do Gladiator (that’s true of any build really) until you get the legendaries you’re looking for. Of course, you can start farming in Challenger, and that will still net you a decent haul. Tankiness should be fine, you just need the top legendaries to buff your damage.
I don’t have experience with Death Knight, yet, so I’ll leave that to someone else to answer. My understanding is that it doesn’t have the best itemization, but can be good. Soldier will pretty much always guarantee you have decent survivability. The damage may or may not be there on Ultimate without the gear though.
EDIT: In the Forcewave Tactician thread, temjiu has a good post with alternate build for more damage prior to getting the Octavius set. See this post.