As a startup, you often have no choice but to build on external APIs or platforms. In real estate, you’re almost always reliant on IDX/MLS data. Or WordPress. Or Drupal. Or Facebook. Or Twitter. Etc. There are, of course, massive advantages to building on top of other systems. But, doing so makes you dependent on a partner who you have zero control over who can change their mind/policies at any moment.
If you’re building, or thinking about building, on top of any external platform, I wrote a post on Medium that you may want to read. It talks about how we (Horizon) designed for a reality that changed (Facebook graph APIs 1.0 going away, migrating to 2.0), and how we dealt with it.