Isobel's first bonus is her 3% damage reduction vs Bastian's 3% increased health in which case the damage reduction is vastly superior regardless of whatever she is wearing be it quickened, vigorous, or bolstered making her effective health higher with the same gear.

Next you have Solar Ward which combines the best of both Drake's Blood and Basalt Barrier, while a shorter duration than Drake's Blood it has no casting requirements and with enough quickened its only downtime is the 1 second cast time. Furthermore Solar Ward scales based on her health so the more vigorous she has equipped the more effective the damage absorption, couple that with a source of minor toughness increases it further to over 5000 damage absorption at its peak. Basalt Barrier's limitation is based on Bastian's attack power, so it is diminished at 3675 with 1h and shield or 4350 with an Ice Staff at the cost of 2900 armor, once Basalt Barrier bubble has popped he also loses out on the bonus healing received at the same time.

2 out of her 3 healing spells can affect her, both of them have heal over time components. While the initial burst from Blessed Sacrament is lower than Kindle, the heal over time makes up the difference sharply providing a steady form of damage reduction and stabilizing her health. Holy Ground is a wide enough area that it is impossible for her to strafe out of it like they would with Ritual of Salvation. Adding Beam of Reproach makes her a hybrid healer/tank, Beam of Reproach's healing doesn't affect her, but it does heal everyone around her target as does Holy Ground.

Gallant Blitz is a default charge so whether she is using a 1h and shield or Ice staff, the option for a closer is there. The AOE Off Balance is also a great opener letting you take advantage of knocking down your target or another nearby if you so choose vs Bashing Bulwark that stuns a single target and may not always work. Honestly I would have preferred Blazing Grasp to pull Bastian to his target than having his target come to him as it doesn't work on the harder elite targets and the sudden jerk away from you on targets it does work is far more annoying when its out of your control.

All of them can tank well and do it differently imo, they're all great in their own way. Bastian is the best as a standard meat shield tank, Isobel is a healer tank, Mirri works as an avoidance tank, and Ember as an AOE attraction tank.

Ember is an interesting wild card, the way I would do it is to use Dual Wield, Heavy Quickened or Prolific. Razor Cape, Savage Instincts, Shared Wards, Spinning Steel/Entomb, and Hurricane Visage. Consider Razor Cape and Hurricane Visage as an extension of the same ability, providing a consistent uptime at the start and end of her rotation. Her Ultimate is the fastest cast and most devastating of them all; getting it up faster lets her do serious damage while at the same time drawing attention to multiple mobs. Not something I would use on bosses though doable, but big trash mob pack baiting if you want to get a lot of stuff together to mow down, she's the one to do it.