Personally, I quite enjoy the trend of simple puzzle games being ported over to Minetest. I think it's a breath of fresh air in a landscape otherwise dominated by the survival-crafting genre. With that said, this version of sudoku is not particularly good.
The problems start at the hotbar. The game represents numbers as physical items, but displays 9 items over of 8 item slots. With none of the items lining up with their slots, keeping track of your selection becomes notably harder than it ought to be.
Due to the vertical presentation of the board, the upper rows are difficult to see or access from the ground. Thus, the player needs to enter flying mode if they want a good shot at completing the puzzle. Rather than enhancing the game or putting a twist on it, the 3rd dimension here only serves to make solving puzzles more of a hassle.
The game has checks to prevent illegal moves, but doesn't have any automatic check to see if you've completed the puzzle. Instead, you must fly over to the block labelled "FINISCH" and punch it. Rather than advancing levels, it then requires you to go back to the level selection blocks, punch the relevant one, and click on the next level. The game doesn't bother telling you which levels are completed either, though it's not hard to guess since they unlock sequentially.
The game suffers from a general lack of polish. Whereas most 'minigames' use the player's inventory as a level menu, this game leaves in the standard Minetest Game menu complete with a crafting grid and craftguide listing every recipe from MTG... There's barely any feedback to moves, and no way to place potential numbers like most Sudoku games do either. The whole thing has serious "barebones proof-of-concept" vibes to it despite having been released 4 years ago and still receiving updates to this day.
I don't think it's impossible to make a puzzle like Sudoku work in Minetest, but this clearly needs more design and development work to be good.
Personally, I really liked the concept of this game, solving a sudoku within a 3D space, which each cell being a node you place a number into. It reminds me of those Minecraft adventure maps that manage to make something seemingly impossible within the confines of Minecraft command block logic.
However... The execution is not really that great. Technically, the game is one (actually, two) mod thrown on top of Minetest Game, which causes a number of oddities such as having access to the entire sfinv inventory, complete with grabbing items from the creative menu (yeah so about Minecraft adventure maps...). The nodes that make up the world are... new nodes, with existing but retextured textures, and the hotbar texture is the MTG 8-slot hotbar even though it's actually 9. Other than these things, the game doesn't make any use of MTG, and is just unnecessary bloat.
There is also a somewhat strange way of handling level completion. When you believe you have finished the sudoku, you need to punch the "Finisch" node and it will then verify to check if your sudoku is complete. Thing is... you can't make a wrong move, and you get all the necessary numbers in your hotbar, so a level completion check should be as easy as simple as checking that the player's inventory is empty, which could be done on every node placement at little cost.
I could go on about other things like how it spews messages into chat, there's no sound or any other kind of feedback when you make a wrong move or win a level, the levels are stored as tiny separate files in the sudoku mod, and that it uses the world folder's filesystem as a wacky key-value store (admittedly this game originally predates mod storage, but it's been updated since then...), but I'm afraid it might get petty.