I have used to go through mirror in 2 LD’s. Unfortunately i ended up in the same room i started every time. Probably it’s because it was my childhood home bedroom mirror and i “knew” what is behind of it. But it’s still fun to step through it and i usually have almost no problem with that. Today at first i was able to put my hand through it, but not the whole body. In second attempt i smoothly went through it: hand first, then my body. I only needed to bow my head, because the mirror was bit shorter than me
What i can say with confidence is: if you are FULLY aware that you are in LD and that in LD you can step through it, you can step through mirror easily. For example i had some problems with flying in LD before (sometimes could, sometimes not) until i started to FORCE myself to fly - “This is a dream and i MUST be able to fly in dream, so i will fly!”. Now i just DO it and it succeeds. The same goes about mirror.
But i think i need to find dream mirror that is not located with my RL mirror, so the element of unknowingness what lies behind it remains.