I've really really enjoyed every singe Boktai very much ... The sun element is stressfull, but you can get good chunks of the game done without the sun.
That being said, Boktai is really too robust of a game to attract people when it has the solar element ... Think Zelda: minish cap or Castlevania:AOS (The Boktai games are that large) only with some device (more emmersive) but can halt your gameplay at moments.
A sun sensor would be better suited for something quick like Wario Ware.
I'm still waiting for something to come along for the game, and benefit you in ways while you're in the dark ... Something to even it all out, and make you wanna get in the sun for additions, or 'bonuses' ...
I'm still a kid at heart, I like new things so I've enjoyed each Boktai .. But I can understand why most people would just want a more traditional experience.
But if you're thinking of trying Boktai II, it's great and the melee weapons / experience are much more fun.