Ok so apparently there's a very harsh limit to bandwidth. I haven't hit it because my Internet is very slow, or something. I haven't tried downloading big files and my upload speed is like 100 kB / s. According to this article I found online, the limit is around 1 gB / 6 hours. I don't care much about that because I'm mostly doing it to back up unimportant files, but you might want to consider testing the limits before compromising.

Another thing is that I use MegaSync. Perhaps the limits are not so hard if you're using that. There's also the possibility that they are but that MegaSync simply makes everything more flexible by downloading files whenever there's bandwidth available. So you can leave it turned on and forget about it till everything is in place. Still, for urgent big file sharing this is certainly not ideal but these are the constraints imposed by using free services.