Older watches, especially those that have been significantly neglected or damaged, may not be able to be fully repaired to be accurate time pieces even with the best possible technician working on the repair. A good watch repair professional will let you know if this is the case before proceeding with the repair.
If you are shipping your watch to a repair person be sure to contact them personally first and ask questions about the service. Ensure that they will return the watch using insured, traceable packaging and that there is some insurance on your watch while it is in their possession.