The down side is that the starting price is kinda high, so the math probably only works out if you also have a number of other workloads to run on the same cluster. Or if you need to run a really huge Forgejo server!
I suspect my comment history will provide the best details and overview of what we do. We'll be offering the Firecracker runner back to the Forgejo community very soon in any case.
* Mandatory code reviews
* Merge queue (merge train)
If you don't need those it's good.
Also it's written in Ruby so if you think you'll ever want to understand or modify the code then look elsewhere (probably Forgejo).