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.
Disabled presence can be an advantage to some privacy-conscious users, so a case for matrix.org can be made for them - especially if they don't chat much anyway.
Is there a dependency on the instance, will it be blocked otherwise?
I have activated the bridge to Telegram and Discord for my channels in the channel itself.