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.

Same skill 1, way better skill 2. The really big thing is the shields will absolutely stop the hit that breaks them, so they can take a Goliath to the face and be absolutely fine as long as their shields were up at the time. Also, you get damage reduction for having more units left. The main thing to note about them is they lose their offensive stats (considerably worse RoF, slightly worse Firepower) in exchange for this tankiness, so you'd better have the rest of your echelon be tanky.