Not OP, but here are the (non-license-based) problems I see with open contribution.
1. It limits tight design. PRs are often going to be for things that the user wants but that aren't on your design plan. Accepting these PRs causes design creep and leads to code complexity, even if each individual PR is written well.
2. It takes time to review PRs. People submit all sorts of code of various quality. Doing several rounds of reviews to get a PR up to snuff (or decide to reject it) takes time out of your day. If your team is small, this is a major cost.
3. It can set the wrong expectations. People get mad when their PR is rejected. Often really mad. Closing 50% of PRs, even for totally valid reasons, ends up with a nontrivial number of people sending you really terrible emails calling you nasty things.
1. It limits tight design. PRs are often going to be for things that the user wants but that aren't on your design plan. Accepting these PRs causes design creep and leads to code complexity, even if each individual PR is written well.
2. It takes time to review PRs. People submit all sorts of code of various quality. Doing several rounds of reviews to get a PR up to snuff (or decide to reject it) takes time out of your day. If your team is small, this is a major cost.
3. It can set the wrong expectations. People get mad when their PR is rejected. Often really mad. Closing 50% of PRs, even for totally valid reasons, ends up with a nontrivial number of people sending you really terrible emails calling you nasty things.