[mmac_]

M.2 and PCIE style connectors are great for board-to-board connections. We use them all of the time in industrial applications. Super cheap and super reliable if you follow the rules. Very common to see all sorts of commodity connectors (M.2, USB, Ethernet etc) get used for non-standard uses.

Don't be afraid to use these edge connectors for hobby work - just make sure to get the board thickness correct and don't stress too much about the gold finger plating. If you aren't inserting too often, then a basic 2U should be fine (you don't need super-expensive 32U etc)

Of course when you're dealing with a user-facing connector, follow the rules and don't cheap out or be lazy.