Question about mirrorless

[Old Crank]

I volunteered for the European games and part of my activities involved work with some media people.
I talked to a woman pro-photog who now has all mirrorless equipment for her work.
$$$$

She says that she thinks it is fine. She misses the sound of the normal SLR.

She also said mirrorless seems to her to be equal to the previous SLRs except in low light situations.

I didn't think to ask her about that observation.
Why would that be? And is it the view finder that is not as bright or the overall performance?

Any of you using mirrorless and can you weight in on that issue.