Workflows that interact with the Github API heavily will fail as they're not available in act e.g. actions like https://github.com/styfle/cancel-workflow-action. Dealing with secrets is also a bit cumbersome. You can throw the following on actions that are not compatible with act in order to skip them:

if: ${{ !env.ACT }}

That said, despite it's limitations, I've been using both act and tmate in combination for a couple of years. Gets the job done.