[dandv]

See the section on "When not to use Meteor" at http://www.meteorpedia.com/read/Why_Meteor

Basically, if you need native MySQL support (some support already exists, but it's not in core), or if you need a web-site (SEO), not web-app.

Regarding scalability, by the time your app becomes so popular to exceed thousands of concurrent users, Meteor will have improved scalability again. They've done it a few times already with Mongo oplog tailing support and whatnot. If you already have a highly popular app, it will take you time to port it to Meteor too. So scalability may seem like a problem right now, but likely won't be one in, say, half a year.