The downside dough is that it's all good when you start, but when you get into more details of your requirements you end up figuring out that it cannot be done in this framework, then you endup doing it outside of it due to it will not meet your needs. OBS: You'll always find some bug that no-one found it till today and you'll be the first guy with that bug once you get there, by then you'll be into 85% of your project. Sometimes you'll find bugs that will be solved "in the future".
If you're into Javascript and JQuery I recommend NodeJS, it's rock solid, very fast and lean JS Framework for Dev, it's server-side. It will certainly be a bootstrap for your project. You can get a hello world running in under a minute, once you get a hang of either Express, SailJS or Meteor you'll have a happy project and time/effort.
Best Regards, Ramon