They are fine provided: you avoid Copper, Zinc, Mild Steel or Iron, Galvanized, or Aluminum. Teflon over the previous is OK but if you get it scratched, you are out of luck.
Also, they are fine provided the give a gap of 5 - 10 mils and widths that are overwidth for the film or paper you are coating by at least 1/8" on both sides. So, a 4" "blade" won't cut it, you need 4.25" to make a good 4x5 sheet of film. The 4" coater can coat a good 4" plate though with proper setup.