Believe what you would like, but I'm quoting facts and figures from one of the foremost authorities on the Suomi and it's history in Finland. The guy owns his own Suomis, has fired them, and is basing his work off veterans accounts and actual documents. As Finnish reenactor myself, I also have had the opportunity to reseach Finnish weaponry, documentation, and veteran accounts extensively. To quote said author, on the manner of firing:
"Muzzle brakes or compensator are useless gimmicks on long-barreled submachine guns like KP/-31. Pressure of the muzzle blast is too low to generate reaction force strong enough to prevent "muzzle climb". A skilful submachine gunner can easily control the recoil and keep the gun steady, by keeping the thumb of his trigger hand behind the breech-cap. This thumb must receive whole recoil force. The butt of the gun is not allowed to hit or even touch shoulder of the gunner. Another way to control the recoil is to keep the gun with stiff hands, again without solid contact on the shoulder."
Also:
"The rear sight is graduated to 500 meters for harassment firing. Aimed shots could hit enemy individuals within 300 meters, but within distances of less than a hundred meters - common in skirmishes in afforrested terrain - the shooter simply pointed the gun in the direction of the enemy without aiming. Most skilled submachine gunners could operate their KP/-31 like a long-range shotgun, without the need of sights. A high rate of full-automatic fire had its uses."
Finally, the weapons testing was done by hand, not by machine. These were army tests, not factory tests, and the results were published in the training manuals to demonstrate was the soldiers were expected to get. Just to emphasize here, here is a Suomi firing at full auto:
https://www.youtube.com/watch?v=gWbIaeomnmI
Albeit it's a collector who is not shooting it the pre-subscribed army manner, it does show just how incredibly smooth and stable the gun is in full auto. It does not jump around, and recoil is easily compensated and stabilized, and this is in the hands of someone in no way trained for months in using it.
AS for the MGs, yes, 3000 yards was an effective range for creating beaten zones. 1500 yards was the more realistic accurate range. One thing to consider as well was that the lafette mounting system was not a modern day army tripod where it's just mounted on a stable platform. The lafette had a series of recoil dampeners that acted like the recoil dampeners on an artillery piece, preventing the gun's recoil from shaking the tripod severely. This lent itself to much higher accuracy. Also, lafettes were all equipped with scopes for long range firing and distance calculations printed on metal plates afixed directly to the lafette itself. Finally, it also had an automatic tracker for sweeping fire, which would automatically angle the gun up and down, allowing the gunner to simply set up pre-detirmined angles, duck behind cover, and simply execute sweeping fire from left to right, as the gun itself angled up and down, allowing for very fine beaten zone firing, especially from reverse slopes.
Also, on the 1000 yard ranges, it was expected to score ~50% hits. Obviously this was in firing range settings from stabilized prone positions, but I'm merely noting just how accurate these rifles were. (The reason many were graduated out to 2000 yards or more was a pre-WW1 concept, in which entire companies and battalions would use plunging fire on enemy formations at range through vollies- the British extended this to the maximum with the volley sights that allowed plunging rifle fire out to 3-4,000 yards. However WW1 quickly eliminted any notion of firing at those ranges). 100 yards with a K98K or Garand is, essentially, point blank. Thet problem was never that the weapon would miss what you fired at, the problem was that the firer was, more often than not, not taking the time to actually aim. That is the situation we wish to see in TW- guns that are on the whole realistically accurate to their specs, however the player him/herself finding themselves in firefights and effects that dampen their ability to coherently pick targets and aim carefully.
To sum up- yes, SMGs will be wildly inaccurate. Yes rifles will hurt and hurt bad. And yes MGs will be something to be feared, and will be the lifeblood of the infantry squad in combat.