GitHub issues are meant for communicating with other team members.
We also use slack so there can be some confusion about whether to ask a question or to post a comment in slack or in GitHub.
Here is a simple rule to remember. If you are not sure about which one to use then use GitHub issues.
Let's assume that on a particular issue there is a lot of back and forth in github. In such cases it is better to jump on slack and resolve it there through a real time communication. However slack should be used as a last resort and not the first tool to reach.
Some people do all their communication via slack. We do not recommend that. Communications related to GitHub issues should primarily happen in GitHub issues.