When we talk about Phase Walk, I don't really mind it at all and being a "skill" class, I find opportunities to use it.
IE: Writhing Horror has been brought up, so thought I would show how I use it there on every CD almost. Start it when just before 1st pull. You do it during tank swaps right after picking up the Jealous male or right after he dies. If there is a boss that you can or champion you don't need to worry about a fontal cone attack, you can hit your aggro button, walk a few meters towards your healers, then run back in optimal position or use it when you don't need aggro or the DPS have to do something else. Then carry on with your aggro building. Seems to work fairly well. Not simple and you have to time it all, but doable. It is a good bonus in my opinion, but one you really have to earn and decide when to place. So, I personally don't mind it and just one more thing to remember.
But, the problem comes from the spikiness and nothing else in my opinion. At least from everything that I know, we are supposed to have the least HP, but also a lot less spikiness to even things out. That is the only part that is missing. Maybe something such as total amount of incoming damage gets mitigated more.. Not enough to make it too much of an advantage, but this would just smooth everything out. Also, could have self-healing based on total incoming damage. Now, in SOLO play, I would love this, but a bit OP. ha ha... But as DogEyedBot said above, maybe a talent that give bonus healing done based on the % of your health. A few options seem simple and viable enough.
The other tanks with high HP were made for the spikiness, but shadows should be mitigating it more, but have lower HP. I honestly would not want just a huge amount of HP as I like the mitigation class. Quite fun to play and less room for error.