In my experience, scaling RoR is totally doable, but very tricky, as the both language and associated frameworks are very terse, opaque and by default seem to do the thing that does _not_ scale well. Because of all this magic, you have to be somewhat of an expert to avoid all the foot guns.

As a new shop, without a known depth of knowledge and talent, I'd probably avoid Rails if scaling is a concern, but if I'm Gitlab or Shopify or some place with world class Ruby engineers, I'm staying the course.