I too have a mixed flock, but was considering moving to Bielefelders for the following reasons:
They do well in my set up (coop and run, no free range)
They lay large/extra large eggs
They are social and people friendly
They are auto-sexing, should I decide to hatch. (This is what drew me to trying them.)
The only downside I knew of going in was that they ate a lot, being a big bird. The big bird part would be a plus should I ever use them for meat.
Well... Things have not gone as well as I'd hoped. The birds are very nice, fit well with my flock, do well in my set up. But of four Bielefelders, they have had the MOST health issues and laid the fewest eggs.
Sunny, one my favorites, died suddenly and for no known reason (I'm going to guess it was a heart attack). Pip, another one of my favorites, had coccidiosis (it happens), salpingitis (related? don't know), but lived 15 months after her first lash egg. Robin has spurs and stopped laying months ago; not sure what is going on with her, as she is otherwise fine. Squeak has had thin shell issues, stopped laying months ago too, and is molting, so she's done laying until spring.
When I get more chicks, sad to say, it won't be Bielefelders. They are lovely, friendly, and two of them have broken my heart this year.