Really fun concept, even the minesweeper side is quite much enhanced by the very uneven mine distribution, bringing up situations rarely seen in normal minesweeper games, requiring different ways of thinking.
The Sudoku aspect works well, solving it often creating interesting new puzzles where the count of mines can break a chance situation on the minesweeper board.
Difficulty is more about mine density, so the 9x9 Sudoku combined with the 27x27 minesweeper is the most challenging, while the largest 36x36 minesweeper remains easier.
Definitely not for someone being annoyed by unfairly losing as that's very common here, inevitably running into chance situations on the minesweeper board (though many such situations can in fact be resolved by elaborate thought, a very nice angle of the game, the classic 50%/50% situations remain).
Would be nice if the generation logic could implement measures to reduce the occurrence of such - still, even as it is, it is fun to play!