I actually think she was far stronger in 2018 before submerge was everywhere. I use Tidecaller a lot and most matches most of my team is submerged and can’t be damaged by her. I have also noticed Frostmage is a pretty popular defence class and that and that can really spam submerge as well and unless you stun the hero there is no way to stop the submerging. I do agree that she is probably the best troop when you need freeze and I did you her in GW today when dealing with a goblin team.
I actually find Krystenax to be superior to Hyndla (except for giant teams) and in most cases Mab as well. Krystenax has a weakness in Submerge like Mab but he still does a really good amount of damage, can get an extra turn and a lot of mana sometimes even enough to refill himself if played right, removes a colour from the board to stop trolls and all these converter troops and summons with his spell whereas Mab can usually do up to 45ish damage if it doesn’t get stopped by impervious, mana shield, wood ancestry, dark ancestry, high ancestry, invulnerable or bless and can get an extra turn if there are enough blue gems.
Khorvash isn’t as strong as he used to be with Obsidius around now but I agree he is still a really solid troop especially being able to stun before he drains mana and does a decent amount of true damage.
I think it really depends on the team you are fighting and what mode you are playing as to whether Khorvash, Mab, Hyndla or Krystenax is the superior troop. Mab will always be the best for fighting looping teams with her freeze trait, Khorvash is usually the best when you have something dangerous in first or second that needs to be drained or if you need stun, nothing better than Hyndla for a giant team and for converters and trolls I prefer Kyrstenax to get rid of gems to stop them getting extra turns or mana. When I do come up with a team that needs a forth troop but doesn’t really need freeze, stun or mana drain I usually just go for Krystenax because he does a lot and it doesn’t really matter if he is blocked on a colour or even both with his enchant.