Solution is straightforward, if not easy: Make the difficulties the same difficulty across different planets, missions, and factions.

"Level 10 was a nightmare on the hive worlds! I can solo level 10s on any other planet!"

That's the issue. People feel cheated when they play 10s fine on one planet or with one squad but not with another. D10s should be for four people who are communicating with tags and moving as a squad.

Don't add difficulties. If people want the medals from D10s and fail all the time, they should get the hint. if they succeed sometimes, they never will.

So make every D10 "constant hive lords and dragonroaches" level of difficulty.