So how do you figure that it sucks? And don't say "Ruby is too slow" or I really will write you off as reciting tired old myths.

Write me off if you like, but here's a short list (just off the top of my head) that we've encountered while trying to scale Rails:

Rails has terrible support for database replication (we've had to roll our own). Its caching architecture is laughably bad, and the implementation was/is buggy (again, we rolled our own), and there's next to no support for the kind of robust page cache expiration mechanisms that are necessary to run a large, dynamic site (Observers are only a start). Rails is a memory hog, takes too long to start, has poor translation and locale support, and simply passing a request through ActionController adds huge overhead that cannot be escaped.

And even though you don't want to hear it, Ruby is slow -- even after working around all of the above, we still had to implement a fast page caching layer just to work around the general, all-purpose slowness of Ruby.

But like I said, write me off if you want.

Seems like you're picking things which are difficult to deal with no matter the language/framework your application code is written with -- DB replication, cache invalidation, etc.