Visa validity is one thing.
Once they enter, they cannot stay longer than 6 months.
If they entered on May 1, and USCIS received extension paperwork after November 1st, they're out of status. If they entered on May 17th or later, they're still in status.
Extensions take long time to be decided, sometimes a year or longer. If they get denied at that time, they won't be able to visit the US in the next 3 or 10 years. Even if extension is approved, sometimes it's approved retrospectively. E.g. by the time they get letter, it's already passed the date they're extended until.