Well from the perspective of say, Bane, weak and broken would be his evaluation. I reckon Bane would have replaced him with a stronger apprentice.
I dont think vader was any of those things to be honest even in his suit. Weak because he turn to the light side in the end? i see it the oposite only make it even more stronger. I can not imagine Palpatine ever doing the same, him being so consumed by evil and the darkside. But Vader was the chosen one though a blatant exception.
The whole sith ideology is retarded though and doomed to fail. A ideology that promotes darwinism, and selfisness, its very interesting but not very logical and prone to backfire sooner or later. A thing that constantly destroys seeking to build something into perfection is a contradictory ilusion. They can gain power, but they are doomed to lose it someday. The problem is the damage that causes in the meanwhile.
I would agree with you that the Sith philosophy is flawed, but it is a surefire way to achieving more and more power and achieving the goal of a Sith - so it that respect it works. I think Sidious and co. were drawing away from the Rule of Two however towards the end, Sidious was pretty much the pinnacle of the dark side and when he destroyed the Jedi the Grand Plan, and therefore the purpose of the Rule of Two, had been fulfilled. Basically Sidious wasn't going anywhere and had no intention of ever being overthrown. Maybe that's why he kept Vader, because Vader was 'weak' (in the sense that his suit had basically capped his power, he could never become stronger than the Emperor) and broken, so Sidious could be Master forever and never fear being overthrown.