Pros
As an intern, you will be working on making meaningful contributions to their codebase immediately and will even have the opportunity to have a say in designing and researching the functionalities you may be working on. Since the company is small, you will be getting daily peer-reviews and tips on best practices from all engineers including the CTO, which is great especially if you want to excel as a software engineer. The technical experience you will gain at Monument is worth any downsides the other reviews have mentioned.
Cons
The environment is almost too open. My boss took a two week vacation a couple of days after my first day, so I had be self-starting and give myself tasks to do. If you're the type of person that likes structured work, then this could be a negative.