A submachine gun usually fires a handgun rounds, even if there are exceptions like the USG-90 (P90). The KV9 (Kriss Vector) even has a modified blowback system that's aimed at reducing the perceived recoil. So it kind of makes sense that as a weapon category these don't usually have the craziest amount of recoil. In Battlefield 6, they are balanced around the idea of being close range weapons that tend to be most effective within 20 meters. This means low TTK, good hipfire and good mobility.
On the contrary, the LMG TTK up close isn't all that bad, but it's not competitive either. In addition to that, the lack of proper hipfire, ADS speed and recoil makes it less effective at those ranges. Beyond 20 meters and especially 35 meters it's a different story, the LMGs tend to maintain a much better TTK than SMGs, but some weapons may still benefit from shooting from a stationary/mounted position to better manage the recoil.
Weapons aren't balanced around TDM and the engagement distances are much shorter, so that naturally means that weapons like sniper rifles, DMRs and LMGs are less effective than their Carbine, SMG and Shotgun counterparts.
