Disadvantages include (this list is probably not exhaustive):

• Presence is disabled (as this uses a fair amount of resources)
• It has a tendency to be overloaded occasionally because so many people use it
• You have to play by their rules (not a problem for most users, but it bears mentioning)

You could also join other Matrix servers, especially ones that cater to a particular interest of yours.

If you go with self hosting, running a decent personal Matrix server that is capable of joining large rooms like Matrix HQ is likely going to cost you money and/or time.