It is vague on purpose. Because if they specify brands, slightly cosmetically models with new brands will pop up. A game of whack-a-mole. We do not want these things falling into the hands of lil gangbangers for more effective drive-bys. Or to arm the marijuana farming gangs in this nations's forests. Or arm homophobic crazies.
I linked to a short video that demonstrates how effective they are. Watch that one.
The problem is they didn't give a definition adequate to let someone consider any given item and decide whether it was legal or not. It feels to me that the concept inherently either is or is not a machine gun, how can "some" of them be machine guns?
As for effectiveness--effective at sending a lot of bullets downrange, yes. Effective at causing mass casualties, no. A mass shooter will be far more effective if they don't blast their whole magazine at a target or two. There's a reason soldiers have burst mode but not full-auto fire!
Automatic fire weapons bear the greatest danger to grouped targets.
Soldiers rarely expect to be going against clumped groups of combatants.
Burst fire is for long range suppression and lane control, to keep people from closing while flanking is attempted.
Mass shooters have a different mechanic, with one of the most disgusting and successful tactics to those aims involving spraying as many bullets as possible into groups of people.