Performance and Scaling of RoR in a Tumblr clone

Is it a good idea to build an RoR app which is similar to tumblr, or should I use a newer stack such as MEAN/MEEN? And what are the issues I might run into in the long run?

The reason I ask is because I read a lot about the scaling issues of rails. Could you be specific in what issues one might have with RoR in this particular case.

Since