I shoot Nikon FF on land for my nature and landscape work.
Underwater, I use M43 (Oly EM-5) with Nauticam, and am getting ready to get the EM1 Mk II. Several reasons for this.
First and foremost, compactness and light weight. A fraction of the FF set-ups mainly due to the very lightweight lenses for M43 such as the Oly 12-50, Panasonic 7-14, and Oly 60mm macro. There is absolutely a huge advantage in weight, bulk, drag, and convenience with the smaller rig.
Second, I travel a lot. Can pack the whole camera rig and lenses, laptop, all filters, batteries, and accessories, in the "personal item" size bag with room left over for dive computers and dive lights, and the entire UW rig (housing with angled viewfinder, dome, macro port, tray, arms, 2x strobes, 2x video lights, all accessories) into a carry-on size roller bag or backpack.
Third, I shoot pretty much equally photo and video. For this, mirrorless (not necessarily M43) is unequalled and far superior than trying to shoot clunky DSLR video squinting at the small screen, switching back and forth between live view and viewfinder, etc. The angled viewfinder on my housing (this is an essential accessory well worth the money) gives a huge and sharp view of the LCD viewfinder of the camera for easy photo and video, and switching between photo and video is an instant push of a button with no need to even take my eye off the subject when things are moving fast.
Fourth, the IBIS of the Oly (even the original EM5) is stupendous for macro photos and especially macro video and the new EM1 ii will be even better. It is hard to overstate how useful this is.
Fifth, the lightweight rig makes the perfect travel set-up. I can add the pany 100-400 zoom (200-800 field of view in full frame terms) and have nature covered on any trip. That lens is so small it is ridiculous for the magnification delivered.
Sixth, no built in flash on the 5DIV, so no TTL optical flash with strobes and the need for more housing penetrations and o-ring sealed electronic sync cables. This is a disadvantage (but could be offset if you get something with a built in flash).
Image quality is excellent, easily printable to 20x30. ISO is very good up to 1600. AF is superb if the light is good, either daylight or a focus light for macro, and is still useable even in marginal light.
In sum, I don't think you really gain anything with full frame (higher useable ISO, and some more resolution) that comes even close to what you gain with the smaller mirrorless system.