It’s about the only way to access home equity without moving out of your home. It does add additional risk and without the right discipline, it can backfire on some people.
That being said, it is what I am doing. Not in rental properties but in stocks (index funds). We just refinanced at 1.6% and took some equity to invest. I have a super long timer horizon (want to leave a large inheritance for my son) and can keep my cool during market crashes.