they are slow and heavy. Specifically they are roughly 85% efficient compared to ~90% for an internal gearbox or ~95% for a derailer. They also add about 1 pound compared to a gearbox or 2 pounds compared to a 1x derailer system.
That depends: if you are at the edge of your range then the Enviolo may well cause you to have to pedal the hard way with a bike that feels as though you're going uphill all the time. I only see one positive about the Enviolo and that's that you can change to low gear at a traffic light even when you're stopped.
