IME the ATEM switchers do not have sufficiently low latency for live image mag use unless you use gen-lock cameras. Whenever I need to do live image-mag, I have to use my Panasonic AW-HS50 switcher.
I own two ATEM switchers, I think they are a remarkable value for the price, and I like them a lot. But unless you use expensive genlock cameras, they are unsuitable for live image mag.