The only disadvantage, that I can think of, to owning your own modem is that it's most likely locked to Docsys 3.whatever (at the time you bought it).
The last modem I bought was Docsys 3.0 years ago (still in use by the kids). *~$120, Arris SB6141 cable modem (Costco item #774054)*
Nowadays Costco sells these two which are Docsys 3.1 modems (not Docsys 3.0) *NETGEAR Nighthawk CM1100 DOCSIS 3.1 Cable Modem* ~$150, Item 1290299, Model CM1100
What's the tangible difference between Docsys 3.0 & 3.1 to the avg user?