The one thing I feel might be problematic is how my screens are scaling to fit my device when in mirror mode. My device is larger than both the iOS and Android size screens we design at, so the preview is not accurate to scale. This makes it difficult to evaluate designs properly.
In this case (for example devise 1200x1600px and design 800x500px) I created empty frame with 1200x1600 size. Then with make symbol from some design(for example design 1) and make instance for this symbol put on frame 1200x1600px(or without making symbol)
then mirror frame 1600x1200px
Or if you have screens: design 1, design 2, design 3
you at first create screen 1, screen 2, screen 3 (each with size 1200x1600) and then put design 1 to screen 1 and etc. After that - created design what you need)
and always use frame with size 1200x1600 for Figma mirror