Please correct me if I'm wrong, but from the wiki, "Comatose cannot be replaced with Skill Swap or Entrainment, or copied by Role Play or Trace, and also cannot be suppressed with Gastro Acid." Therefore this common strategy may not be a check to be considered.Don't forget Skill Swap and Entrainment can stop the strategy. Having a Mega-Gengar with Entrainment Normalize supported by Sticky Web means the Comatose user has to be both faster than Timid Mega Gengar and carrying a Choice Scarf.
Also adding that the relatively recently founded set, Flame Orb Quick Feet Pokemon can have some possibility of outspeeding oponent's Comaphazer (Non-scarf Deo-S). Although this may not be the case when they have scarf mons with a higher base speed stat than the quick feet mon.