What property management software is scalable with an open API?

I’ve called all the usual suspects: Appfolio, Yardi, Buildium (Real Page), and Entrata. Only Buildium provides open API compatibility with their service. The others won’t do it unless you’re an extremely large institutional player (10k+ units).

1. Any other softwares I should look into that are scalable, or should I just sign with Buildium? Would like something that you can scale to a few thousand units, so not open to the more retail providers.

2. I only have experience with Yardi and Appfolio. Anyone have experience with Buildium? Is it fairly good?