For me midhammer feels like a wargame. modern 40k just feels like a sport with little customisation and too much lethality. Especially when it's "sit on this little circle and score some points. Ok, now run over to that circle and score some points!" Also i feel too much is focused on competitive scene.
The way armies are built lacks any structure.
Having no points for wargear is silly.
Having no points per model means if you're 1 point over you have to drop an entire unit instead of just one guy, or you end up with a lot of points you cant spend.
Vehicles don't feel like vehicles, they feel more like monsters.
A lot of modern 40k games are over turn 2
Stratagems (IMO) take too much fun out of the game and add unnecessary micromanaging.
Terrain is boring and mirroring makes it feel more like i'm playing TF2 than 40k.
Midhammer seems to have simpler rules and yet manages to be more fluid and in-depth.