If you unlucky, the engine block or cylinder head might have hairline crack, then after skimming cylinder head, car will drive ok until you hit heavy traffic, then you will notice temp gauge go from 1/2 to 3/4 or even further, blow another cylinder head gasket.
Sometimes mechanic did crap job with gasket or fitting cylinder head, sometimes company that skimmed cylinder head did crap job.
Best is to take the job to people who specialize in engine overhauls, not your average mechanic.