Well with tametsi some of the boards don't have homogenous shapes. Some of them do, but with others you might have a mix of rectangles and squares, triangles and hexagons, or octagons and rhombuses. I actually misremembered the amount of shapes there are. I had to go on google images to confirm how many shapes are in the game.

They're pretty similar, but Tametsi has less rules than Hexcells, apart from one rule where the tiles have different colors and it'll tell you the total amount of mines as well as the mine counts for each color. Hexcells and its sequels have some brutally difficult levels themselves, but the thing with Hexcells is it uses its "rules" a lot more frequently, and it does allow room for error. In Tametsi you can make errors in the levels, but you cannot make a single error if you want to actually mark the level as complete. IIRC in Hexcells you can make three errors.

Plus in the end levels of Tametsi, there's not really any clear jumping off point for solving the puzzle. There might be some mines that look really obvious, but you might mark those and not have anywhere obvious to go from there. With Hexcells, that can also be the case sometimes, but more often than not the path sort of "reveals" itself as you solve more and more mines.