DISCLAIMER: I work at CARTO as Support Team manager and Solutions Engineer
CARTO Open Source is equal in functionality to an Enterprise paid account, but apart from having to install (and more important, maintain) your own installation, there are differences in a few services.
Routing, geocoding, and any other Location Based Service on a paid account come from HERE. You should have to set up your own HERE (or very soon Mapbox) credentials to get the same functionality.
Data Observatory is a separated component (shared by the full platform) that you should have to set up on your own (it's also Open Source).
We (CARTO team) do our best to answer here but Enterprise accounts have the support you may expect.
There may be other minor differences but I think that covers the relevant parts. In essence, carto.com tries to be the easiest and more convenient way to get access to the technology we develop in the open.
There's another relevant difference. CARTO provides separate Enterprise pricing plans for BUILDER (the UI to generate dashboards) and ENGINE (to create your own custom applications). That difference is not present in the Open Source version where you have no limitations in the use of API keys.