Metal & Plastic you've painted yourself yes. Pre-painted plastic (i. e. Heroclix) no.
For things I've painted myself (metal, resin, polystyrene plastic, PVC) I've found that de-greasers work great. If you're in the US, you can find them under the brand names of Purple Power and Simple Green. They make short work of brush on paints and primers. I usually will soak for a day or two, then scrub with an old electric toothbrush. Paint in the crevices will take the longest to loosen up, so sometimes it takes a couple rounds of soak and scrub to get all the paint off. Beware that these de-greasers will sometimes de-bond super glue joints, especially on unpinned metal joins. I've never seen them soften up a welded joint (i.e. polystyrene model glue).
For spray paint and primers, it's hit and miss. Some spray paints and primers will come off easily, some I've soaked for a week and still not gotten them to budge, so I'll usually strip down to the primer and restart from there versus going all the way down to bare plastic.
For pre-painted minis, I've never found a good way to get paint off without ruining the figure. I've tried the de-greasers mentioned above, Easy-Off oven cleaner and even break fluid. Those will make the mini super clean, but don't seem to make a dent in the paint. Acetone will take off the paint, but it will also dissolve part of the plastic itself, In the best case scenario figures get all soft and floppy, in the worst case scenario they just melt. I've just learned to live with the thick paint and primer over it.