We've relaxed this one, currently, but it could change. Some commands simply do not work inside dropdowns - we have a line of explanation text about that.
Notably, things which are already dropdowns do not work inside dropdowns. These are denoted by a little down arrow. These include the font dropdown, and paragraph format dropdown (which was allowed previously, but shouldn't have been which is now fixed). Dropdowns look like this, so you just can't put dropdowns inside other dropdowns.
The other things that sort of look like dropdowns when you click on them including text colour. insert link, insert image and insert table are actually called "popups".
Insert table was being blocked from being added to a dropdown on the basis that it is a popup, but that might have been slightly overzealous and we weren't actually preventing other popups from being added to dropdowns so it was also inconsistent.
For now we've allowed the insert table popup to be added to dropdowns but it has to be said that the behaviour of popups inside dropdowns is not optimal, IMO. They work, but upon clicking on a popup inside a dropdown, the dropdown closes and the popup opens were the dropdown item you clicked on was.
This may look odd, therefore, but at this point we'll let admins use their own judgement.
Personally, I wouldn't bother putting popups like insert table inside dropdowns and I'd look at a different approach, but it's up to you.
They aren't. The insert table button is in the "More options" group section. That's different to a dropdown. It's basically a second toolbar in its own right which buttons overflow to when the group they belong to contains more buttons than their configured limit.