The problem is when people then open huge PRs and expect you to take time to review them, then eventually merge them.
Especially when it's something you don't want in your codebase because it introduce a big unnecessary "refactoring" or a feature that you don't want to have to maintain forever.
The point is that saying "pull requests welcome" is still work for the maintainer, because now you have to have these discussions with potential contributors, sometimes explain them why you don't want to maintain the feature, or explain them why this PR is not the way you want...
So either way it's work, it's important to keep in mind before saying "just send a PR".