Barachiel provides some good examples... just not for the side he's trying to argue. The following assumes no rebuilding, but a DM that plays enemies to their intelligence.
Storm Giants are difficult to control due to constant freedom of movement. A 15th-level party is going to be facing them 2-4 at a time, and they are smart enemies. Add both Control Weather and Call Lightning to the SLAs, and they can nuke out weaker party members. Not too bad, though, if you are prepared.
The Pit Fiends have Wish once each. That means an unimpeachable teleport, without any deviation. Vitalist finds himself at point-blank range to three of the nastiest monsters the Nine Hells can conjure up, two of which probably readied actions to do horrible things to him. They also have a wide variety of SLAs, SoL effects, plus hit like trucks anyway, and are smart enough to focus fire and one-round major threats.
This is a roundabout way of making the point. A party with a Vitalist is virtually immune to HP damage unless it is enough to one-round or one-shot the character in question (Guardian Method makes this quantity much more difficult to reach for enemies). However, no amount of healing or temporary HP can stop a save-or-lose from connecting. They are excellent buffers and party support, but any DM worth his salt can present threats that are still threatening. They are only overpowered if allowed to be, something that takes actual effort once past level 10 or so.
