Ok, so I have been following the threads lately about Shadow/Assassin spikyness, and I understand that in all end-game content they are more spiky than guardians/VGs. However, at which content tier does this cause a significant problem?
Shadows suck as tanks from 10 until your 30s, because, for those levels, the only thing you've got to mitigate incoming damage is your armor, and that *sucks* compared to what Guards and VGs get. Of course, the requirements for tanks are really low for those levels, so you'll not likely notice it.
From your mid-30s until 54, you'll notice amazing performance, especially when you learn to use the tools given to you because you finally start getting a nice itemization budget to make up for your terrible armor on top of *finally* getting access to a vast majority of your self healing.
At 55, when you're just starting to do content, you'll be in for a rude awakening. For the hard modes, if you start at appropriate gear levels, you're essentially going to be *forced* into using and abusing your CDs in order to avoid falling over dead for no reason, especially on the really *bad* trash packs (dog packs will be your eternal nemesis). The spike problems just get worse the higher into content you get. Keep in mind, all of this refers to doing content in the *intended* gear level. If you overgear it a piece of content, you're not likely to see any problems (or at least not as extreme).