I have yet to use sandboxes though have some experimentation planned for a new gig I'm starting in a week.

How would you compare what you do to fly.io's Sprites (which is currently atop my To Try list)?

Similar to Sprites in capability, so if you can use sprites you can use agent37. We're also about 94% lower cost than them.

Fly certainly has great branding since they've been around for a while now. So they have a distribution advantage. But I think at some point they will also need to own some niche rather than just being a generic sandbox company, given how competitive the space is.