this is what is in the documentation:

Can I use MySQL instead of PostgreSQL?

Yes, but we are recommending to use PostgreSQL to ensure long-term compatibility with OpenProject. Some features such as full-text search are only available on PostgreSQL, and we may decide to reduce or drop MySQL support at some point in the future.

I choose OpenProject because it supported MySQL and I can live without the full-text search support. Now, after using OP for a while and browsing through the forums and the Work Packages, I am wondering who this "we" is, that will decide on future support of MySQL. This is still an open source project, isn't it? 

Remark: Unfortunately I am not familiar with Ruby on Rails, otherwise I would take care of MySQL related issues.