The buffs don’t stack, the enemies are just going to be higher level and more upgraded to make it harder. So in a way you could say it is stacked as a ruby+5 venom will be x multiplier compared to Platinum+5. In terms of what is listed as buffs however no, the enemies are just stronger is all.
The missions named Hard, Brutal and Master definitely earn their name. They are meant to be challenging and if you don’t have the right mixture of heroes unlocked and upgraded it can be a real struggle to beat Hard 1, most small accounts won’t have the desired heroes and those heroes won’t be upgraded enough so they will likely just stop at Medium 5, this is why they made that mission replayable as it is the cutoff point for a lot of players.
I used Kurtz and I am pretty low in stars, but still managed to beat it with 1-3 heroes due to Kurtz’s health suction, but yeah my Razorback Plat 5 10 star Lv.91 instantly melted and you would need to automatically get his shield up.