I'm thinking that they may not be as effective as a 2-flute cutter at plunging since with only one flute cutting (instead of one and a bit) the direction of the resultant force in the X/Y plane is going round in circles. Whereas with 2-flute I imagine the the forces are almost equal, and they are opposite ... so they should at least partially cancel out. That does not take into account swarf ejection though...

We used carbide 2-flute cutters (2mm and up) on the little router at school all the time with various plastics. They lasted a long time.