The architecture and flexibility of Platform.sh is up there with the best. It's great to be able to use composer to manage dependencies and have a containerized system that builds those dependencies on each deploy. It's what you, as a developer, would build for yourself if you had the time. But really it's better because you don't have to build or maintain it.
There isn't much that can't be done, even compiling your own PHP extensions in your instance!
The documentation is thorough.
We started using Platform.sh very early after they opened their doors. There have been growing pains, most of which have been resolved. The weakest points right now revolve around visibility and analytics into what your instances are doing. An example is that we've had sites run out of disk storage with no warning in the past. Although, more recently they've implemented an email warning about low disk space to resolve this issue. But analytics within the dashboard about disk storage would be even better.
Support is also a weak area, especially involving followup. The initial response is fast, but if there is any back and forth it seems like the ticket gets lost in the shuffle. The best way to deal with that is to jump into the Slack channel and ask for some attention on your ticket.
Despite these criticisms, it's still one of the best and most flexible systems we've ever used. And they are always improving. Most of my past pain points have been dealt with over the years.
We use Platform.sh for client Drupal websites with moderate to high traffic and high dependability.