Difficult to use? No.
Easy to knife-kill a single enemy? No - but I can shot them as I run in then use the knife to finish
Are they effective? Yes.
Modified? No.
Easy to kill multiple targets? Yes. No easier than with other weapon combinations, but they don't make it harder either.
Only the Tampa can really compete with a pistol for reload speed, and many people think of that as an oversized MP anyway, so they've got a good edge there. My main tactic is, as a Light, to have the MP as a secondary. If I'm mid-fight and run out of ammo on my SMG (usually a Galactic with high Capacity Mag - it takes quite a lot to run out), I'll switch to my MP and blaze away - I'm usually up-close with the enemy I'm firing at anyway. If there's a fresh opponent coming into the fray as I switch, I'll rush them while firing, then knife them to finish. I don't often sneak all the way in - even if I'm short enough on ammo to be staying with my MP instead of the Galactic, I'll be opening fire before I strike - hoping to chip away enough health to incap with a single knife-strike.