I'm happy with Varangian Harald. I view him as the opposite of Basil II: he starts off extremely strong, but his abilities lose overall power throughout the game. That's okay, though - by the time his abilities decline, you should have already snowballed hard.
Compared to Basil II, who also snowballs hard but his combat strength and debate strength keep getting stronger and stronger - and walls are ubiquitous late game, so tanks being able to blow away walls is a very strong feature.
Also like Basil II, you should approach domination in a very particular way. In Varangian Harold's case, the emphasis should be on Suzerainty - the varied yields that defeated units produce via levied units can often be used to help complete quests that city-states give. Combat units produced by Harold himself (you) can often be limited to siege and ships - perhaps a smattering of ranged units as well, to soften up targets for levied units to kill.
Finally, I'd like to note that the units which some City-States can create are no joke; especially if they are supposed by, say, trade routes and given space to expand! Considering Varald (just roll with it) is the only leader that can potentially levy armies for free (!), this allows him ample defense to pursue a diverse degree of victory types.
When I first saw Peter the Great (my first love) I knew immediately he was powerful.
When I first saw Basil II (my main) I knew immediately he was powerful.
When I saw Theodora, Ludwig, Tokugawa, and Yongle I knew immediately they were going to be powerful.
But Varald had me on the fence until I started playing him on his own terms instead of trying to force a generic "domination" playthrough on him. Like Basil II, there is a particular approach that works with him -- if you master it, I think he's far stronger than what the "average" player thinks -- and to be clear, I don't think people posting on this forum are average players.
This is why some people put Basil II very low on their "tier list" and many expert players consider him one of the best domination/religious civs in the whole game. Playing Basil II is atypical.
I think, in time, the same will be realized about Varald.