This very same question does come up evey now and then, on several different forums. Alot of people have claimed damage to the motors used with them, and many will say its because the static unit only delivers 2 thirds power.
Really, I dont see any problem using one, unless your running the machine hard, and need the full HP rating of the motor, and using a static converter you simply wont have. If its a mill you have powered with this converter, then I wouldnt think you would hurt the motor. Milling machines seldom see full load to the point they use all the name plate HP.
I have also read others claim the static converter causes the motor not to run as smooth. I can not comment on that, as Ive never used one. I guess its that old argument that your running on 2 of the 3 phases.
When its all said and done, motors can crap out when ever. Regardless if run on 3 phase off the pole, RPC, or VFD. Many do claim the motor life on a static converter is shorter, but its allways a hear say thing most of the time. If the motor your running is just a run of the mill motor, nothing special or exotic, then I would just continue on running it. If you have a failure, then have it rebuilt, or replace it, and then decide if you want to change your ways of powering a 3 phase motor.