It´s very nice to have the iniciative of learning al language by themselve, and certainly there is a lot of benefits considering the resources outside to make it. In terms of advantages, there is a LOT of pages, apps, software, etc, to learn a new language by yourself, and if you are consistent, you will improve in vocabulary, grammar issues, and you can familiarize with this new language.

There are disadvantages, indeed. The first, if you do not practice the verbal and conversational skills frequently, you will have a roof. You can finish all the language apps out there but you won't improve to the next level if you don't speak the language with others, and there is the hard part. Finding a language partner is not that easy, you chat once or twice with a new partner but after then mantaining the regulariry is really hard.

I started myself to learn chinese 7 years ago, and then I studied three semester in an online Chinese University, and I can tell you in that period of time I improved my skills 10 times more than all the time I studied by myself. Good luck with the french, I'm learning it too right now but of course is much easier for me.