Here's some places I noticed it:

> critics love the Twitter example. But look closer. Ruby carried them further than most companies will ever reach. They outgrew their shoes. That’s not an indictment… that’s success.

> I’ve never seen a team fail because they chose Ruby. I have seen them fail because they chose complexity. Because they chose indecision.

> GitHub held the world’s source code together for years using Ruby.

There are many examples of companies that used Ruby at one point very successfully but moved on from it once it no longer fit their situation. This isn't a critique of Ruby! But it is agreeing that Ruby can be outgrown and that, if you are looking to start with a language your usecase probably won't ever outgrow, Ruby might not be the best choice.