Blue alone is well known for having no shortage of solid cantripping creatures, and so there's plenty of ways to draw cards
There was a lot of talk about how a lot of the cards were mechanically unique and not easy to get when this was announced, which meant prices were probably going to shoot up
The three most useful recent additions to equipment decks in EDH Ardenn, Koll and Reyav all share Halvars abilities between them, and redundancy is the key to any strong strategy
Senu's last ability will trigger only if Senu is already exiled at the point immediately after blockers are declared and at least one legendary creature you control that attacked isn't blocked
In retrospect, it completely makes sense that a trading card game based on Disney characters would be a massive hit
Frankly, I've never ultimate-d him, because I'd always rather kill something or draw cards and drain life from various effects, but it's nice to know that you have a big effect that could win the game in one hit