For all those seasoned devs/eng leaders out there, what are your heuristics for when it is a right, or wrong, call?
In the past, I've hired a dev shop/freelancer when the project could be worked on fairly independently with high level direction and has a strong business use case but it isn't worth wasting core team cycles. For eg, marketing team needs a dashboard, the reporting/ETL infra needs to setup or is broken, internal products, or even clearing the bug backlog. Ideally the dev shop also has someone who can do some product/design work if necessary.
If you're hiring a dev shop for higher velocity/shipping features quicker, you have to be careful. Reviewing their PRs, communication (if in different TZs), sharing context, poorly done work, and disconnect between team's best practices and theirs can eat into your core team's time and potentially lead to some frustration.
As much as possible, the key is to protect your team's time aside from critical inflection points -- initial scope, them sharing their approach, validating if it looks good, and the occasional review/check-in.
Hope this helps.