It's hard to think of a gimmick more ill-conceived than Boktai's "solar-powered" one. Essentially, there was a sunlight sensor on the actual game cartridge, requiring you to play the game in direct sunlight in order to charge your weapon in the game. There was probably a few thoughts behind this gimmick:
It's that third point that was the major problem - to charge your weapon and play the game as it was meant to be experienced, you pretty much had to force yourself to deal with glare (which the Gameboy Advance did not handle all that well). Not only that, but if you ran out of weapon charge in the middle of the night, you were stuck playing the game in panic mode, hiding from enemies. Not exactly an ideal way to set up a game, but at least a lot of frustrated gamers probably got sunburn.