Otaiken

As a side note Dagath dark mage build with Incarnon Ceramic Dagger and Grimoire is some of the most fun I've had in this game. It's just an unceasing rotation of massive damage and particle effects. Is it more effective than cracked Mesa or Saryn build? No. But oh boy is it fun to trample enemies with spectral horses in between throwing "fireballs" and spectral daggers. The Grimoire and Melee Exposure were made for this frame. Now I just need to find a fitting primary for this. Is there a decent spear gun for SP+?

Dagath is certainly strong. She also has extremely good passive damage buff. But getting her to the point where she is comfortable to use in SP+ is pretty expensive. For me that point is Vitality + Adaptation + Arcane Guardian + 2 health shards (Tauforged for max overkill). This makes Dagath comfortably tanky in SP for me. I also use Incarnon Ceramic dagger with Life Strike to instantly heal myself. I personally hate shield gating on Dagath thematically and mechanically. With that I have a frame with great passive buffs, great survivability and decent nuke potential. I also don't have to spam abilities like crazy just to survive. But looking at the investment it's certainly not cheap and accessible. I made that investment for the sake of variety because I'm getting bored playing other frames but it's not like I would recommend an average player go and get her asap as their first generally powerful frame in favor of frames like Protea, Mesa or Saryn.

It helps all of them. Obviously melee weapons which already had strong heavy attack builds benefit. But then there's weapons that have heavy attacks which are good, but not worth building for. For example Venka Prime (Claws in general) has a very strong heavy attack with a slash proc but it's a forward stab. So even if it is very good for obliterating a single tough target in the middle of a group, you will never build for it. With Tennokai you can actually use it for its inteded purpose now. It's a similar story with all weapons with single target focused heavies.