Facebook closed groups

I have noticed several niche online training products adding a bonus of a closed facebook group for interaction.

When you buy the training course you send an email to the owner who adds you to the group.

This replaces the idea of a protected forum you often find inside online courses, which in some cases don't ever catch alight.

The groups I have seen are incredibly active with post every 20 mins, people sharing content, links, videos. Reason is people are logged into FB all day in many cases and it's seemless to drop into the group and see what's going on without opening a new site and logging in.

Downside,  as a site owner you could get booted off FB and lose all the content shared in your group plus though searchable the content is very much a live flow of latest info and old threads get buried.

Any thoughts on the pros and cons?