In my last company we were burned by these "IT firms". It is basically impossible to get any meaningful work done unless you have a person on site with the dev

In my last company we were burned by these "IT firms". It is basically impossible to get any meaningful work done unless you have a person on site with the dev team. You also need to write very very very detailed design docs and feature requirements. If you absolutely must use services of these companies, try to get them work on peripheral features, instead of core application logic. You'll be minimizing long term technical debt damage risk to the project that way.