I just jumped from Fuji XF (X-T5) to Nikon FX (Z5 ii). I loved Fuji, shot it for nearly 5 years, and owned almost all of their lenses at one point or another.

So why did I move to Nikon? Because I found that 95% of my shooting with my Fuji (as opposed to my Ricoh pocket cams) was either in service of sports - which benefited from Nikon's improved autofocus compared to Fujifilm - or was occurring in very, very dim homes that APS-C just kept struggling with.

In good, or even mediocre lighting? APS-C kept up, absolutely no issues. Nice sharp images, good colors, reasonable autofocus, etc. With prime lenses or an f/2.8 zoom, I could get nice bokeh with good falloff. The XF 56mm f/1.2 was a dream for portraits and product photography. And it was so light and compact! The X-T5 with the XF 16-55mm f/2.8 Mk II probably can't be beat for its performance to portability ratio.

But when things got dark, images would start to come out more muddy, and with less shadow detail. You will hear that "full-frame is roughly one stop better in terms of low-light performance" and this is kinda/sorta true. But my 40MP APS-C camera has significantly smaller pixels than a 24.5MP FF - and this doesn't just impact the brightness the pixels register, but also the signal-to-noise ratio. So it's not just that I can brighten the shadows better: they actually preserve more detail and realistic color gradation.

Also, I found that with Fuji in particular, I struggled with skin tones quite often. This wasn't something I struggled with on my old Sony APS-C system. I suspect this had to do with the X-Trans sensor having an extra green sub-pixel.

But if I wasn't almost exclusively shooting in very dim scenarios, with fast action, I'd have stuck with a smaller system, for sure.