1: The Magal is also a unreliable weapon. The problem that I have read about, is that they made the barrel too short for sufficient pressures to reliably drive back the bolt group.
2: And I have heard of people having to empty an MP-5 into a foe to drop them. The other problem is, the .30 Carbine round used in Korea and Vietnam, was still using WW2 manufacturing technology. And again, complaining that it has taken 6 shots to kill a person with .30 Carbine? Sounds like Shitty shooting (You can shoot a man 6 times in the legs with ANYTHING and he'll still be alive). Complaing that it takes 2-3 shots is again, comparing the Carbine to a full scale battle rifle. Think of it as an SMG and it makes sense. (M2 Carbine for instance)
3: There are also Spire points being made in 110 grain weights specifically for the .30 Carbine. If loaded to a better velocity, and using Expanding Full Metal Jacket technology bullets, or even just regular FMJ.
You brought up the 5.7x28mm round which has an M.V of 2,345 fps, with a 31 grain bullet that puts out around 400 ft-lbs of energy, and has a wound channel repeatedly compared to a .22 magnum.
Great for going through armour, that has been repeatedly proven. The question however does remain, of it's terminal ballistics
10x25mm or 10mm in the MP5/10 or MP10, however you wish to name it....well, asside from it being an aborted run, and not even being chambered in the UMP series, the reason it was dropped was because of, recoil. The round recoiled too much, at the rates of fire being used. While I will not disagree that it will out perform the .30 Carbine against an Unarmoured target, that is plainly obvious. However, One needs to wear NIJ Level III armour to reliably stop .30 Carbine, even in it's normal loading. Unfortunately, I haven't fond any references to the 10mm's ability to penetrate armour other than it's offspring, the .224 BOZ.
On a final note, In very roughly the same length of a magazine, you could fit an extra 5 rounds of .30 Carbine. Or have a shorter, more compact magazine (in comparison to 9mm).
Or 40 rounds in a magazine of near equivilent length as a 30 round 10mm magazine.