Serif fonts are typically harder to read on a screen than non serif fonts. All those extra thin horizontal lines constantly make me second guess letters unless I'm right up on the screen. It's especially bad when the vertical lines are also rather thin. However a good bold serif font can be just fine (the Tested logo for example), though that's rarely the case with Japanese games.
There's nothing wrong with serif fonts. Depends on the application.
For some reason thin serif fonts are not as difficult to read in actual print. Maybe it has something to do with the dpi.
Merely a pet peeve of mine.