Sustain is good on a Sorcerer as you can build a high Magicka Recovery build and then you simply use Dark Deal to convert Magicka into Stamina.
The Clannfear heal is a heal based on your Max Health which means if you have high Health then your heal will do more healing. When this heal crits it can easily heal 30k+, and can be buffed even higher with the use of Restraining Prison which buffs you with Major Vitality. Restraining Prison is extremely strong on boss fights since you still gain the Major Vitality but you also get almost a full refund on the cost of the skill if it doesn’t immobilise anything which is exactly what happens when you use it on a boss so its an almost free buff to your incoming healing.
Sorcerer Tanks are really in their element in trash pulls and for things like vAS Off Tanking because they have skills such as Ball of Lightning and Boundless Storm which means they can move around extremely quickly.
Sorcs have the option of Bound Aegis as a panic button, it only lasts for 3 seconds but if you time it with hard hitting mechanics it makes it very easy to survive since it gives you 40% damage mitigation which is huge.
In terms of group utility, you have a few useful group buffs which really benefit a Magicka group, and an important skill for Stamina group to reach the pen cap. They can easily provide good AoE Elemental Catalyst because of the Shock Damage skills.
The only major downside to the Sorcerer Tank is crowd control is not as good as the Dragonknight, Restraining Prison is not bad but having a 360 degree immobilise would be nicer and having a Magicka based pull skill would also help a lot.
Choosing a Sorcerer Tank is a very good idea because they are the full package when it comes to Tanking. You can play both as a Main Tank and an Off Tank well because you are able to take a lot of damage, heal yourself well and provide a range of group support.