We have a pair of VM phones here.
The only problem I've had is coverage. It's the same as Sprint and about your second point. It's never slowed or stopped here but let's say someone that didn't know better tried to use that to replace the home internet service. Yup, they would be upset as that connection no matter which supplier seems to wink out every hour or two.
That appears to vary with location as it does it along the west coast from Tijuana to Vancouver (just north and south of those places.)