While he undoubtedly has drip (just look at all of those beads, and the golden kneecaps carved to look like dragon faces) he also has a bit of a branding problem
If you have a way to cast a card from your hand without paying its mana cost, you can fuse it for free
Milling your opponent can be playing right into their deck's game plan
Cards such as Overwhelming Stampede and Overrun can give your army the extra power it needs to eliminate a player, while Heroic Intervention and Tamiyo's Safekeeping keep your key critters safe
You then decide to play Panglacial Wurm - you can play it now mid-searching, but you cannot use the land you want to find with Mesa, since whatever the search effect was, has to resolve after you're done with Wurm shenanigans
Seahunter Seahunter is another merfolk support card that works wonders in its intended deck, yet falls flat anywhere else