They have an HP shield, and at 4 stars can reduce damage taken for both themselves and allies in the same row. What's not to like? Surprisingly good (61) Evasion at Max, and good Armor (more than a typical SG), and honestly their main issue is relatively low damage and being melee. Which is not really a huge issue for tanks on the front row. Aside from that, theyre basically mostly SGs that beat things with sticks.
Do note that unlike SGs, you cant easily raise their stats with things like formation buffs, fairies and the like.