Xiao's berserk buff only buffs her atk on normal attacks according to the wiki. I guess if she berserks (which is also a taunt), it will buff her counter-attacks, since those are normal attacks, but her defenses are terrible. Xiao won't be a good dps, but that 40% atk/def break is excellent if you don't have it from another source, and she's worth slotting in for that alone. Ling or Elza both can do better today, and WoL can do better in a few weeks.
Ling buffing her normal attack by 100% per turn is pointless. She can only equip daggers, and has a garbage base attack with no passive bonuses at all. Like Xiao, she has excellent 45% breaks, and that can justify her slot on a team even though that's really all she brings that matters. Again, she's obsoleted by WoL in a few weeks.
Yun is pretty good, with great stats, a ton of passive atk bonuses, some immunities, and a good chainable attack, but has a bad weapon selection. I don't think he's worth chasing, since he's not as good as Orlandu and that's right around the corner. His TM is awesome, and as a limited unit, you either get it this week or never, so that's something to consider.
I'm going to evolve the Xiao I have to 6* to benefit from her 40% break in the short term, but I'm not going to pull on this banner since Ling has a very short lifespan and Yun will be too rare (and with his weapon selection, he can't ever be "great" without Katana/Greatsword masteries).