I really enjoy the killer sudoku, but the engine that drives it is unfortunately absolute garbage. I suffer through it, but the gameplay gets tedious fast because of the engine. The settings seem to leans towards either constantly filling and changing the cell candidates I’ve chosen or becoming tedious by not removing obvious and trivial candidates. Like for killer sudoku it automatically fills candidates up to and including the number being summed; e.g. the total for a zone is 7, it removes 8 and 9 but not the 7 itself which can be trivially excluded from any zone with more than one box. If I left the game automate too much it mangles the candidates I’ve already selected, especially if I ever need to use the undo option because I inevitably fat fingered a digit. Haven’t found much better on mobile online though, especially for killer sudoku.