Personally i would never put any painted article through a thicknesser, i have done it but the risk of ruining the blades is too high, apart from the paint dulling the blades you WILL hit a hidden nail or screw sooner or later.
If someone brings you a piece of wood to thickness & says "I have checked it for nails" you can almost guarantee this will happen!
A coarse sanding disc on an angle grinder is also good at stripping old paint prior to planing.
Fair enough. It's down to judgement. I wouldn't bother putting cheap painted pine furniture through the thicknesser but occasionally come across a quality piece that has been painted. I check it thoroughly for metal with a detector and skim it a little at a time. This corrects any cupping the board as well. If the blades dull ( I haven't particularly noticed this ) I have a Scheppach sharpener with a jig to refresh them. Again good extraction with a cartridge topped extractor is essential. This probably takes fifteen minutes compared to a day of hand scraping.