The paint on sprocket mounting surfaces will compress in usage and will make sprockets loose or the sprocket may not fit on at all!
If you are not replacing the bearings, bearing surfaces should be masked or be prepared to spend some time removing the paint! Even when replacing the bearings, built up paint can hamper the pressing in of bearings.
Bolts will not fit in the threads or nuts will not thread on to threads!
When assembling the electrical of your bike built up paint can give you a faulty grounds.
Not to mention the incidental error of having the brake drum inside powder coated (right Roger) and the hours to remove the paint!
Brake Master Cylinders, Brake Calipers, and any holes (threaded or unthreaded) in the frame or tin need all fit in to taken into consideration when powder coating.
Care should be taken to address these problems to the person doing the powder coating. Do not assume they know what they are doing! Better yet, mask the parts prior to dropping off the part! Electrical faulty grounds are addressed when assembling the electrical parts. A little scraping of the paint around the connection may be required. But removing the paint from inside the brake drum is no fun!