I love my mirorrless bodies (A9 II and A7RIII, plus soon to be A1). I'd never go back to shooting on a DSLR ever again just for the sole reason of having an EVF (electronic viewfinder). No more fumbling and guessing at settings in the moment (like when a tornado is on the ground), I can see the photo I want right as it gets shot and often times the ability to shoot at 20+ fps until my memory card fills up and not have to worry about whether I got the focus wrong or what not.
My Sony glass is many times sharper than anything I ever used with Nikon or Canon as well and despite many claims of poor battery life, I can shoot all day (birding and sports) on just over 1 battery and easily do 2500+ photos (Sony A9 II). The autofocus on the Sony full frame cameras (especially the A9 series bodies) is faster than anything I've seen in my life. I have a 95+% keeper rate on many of my bird photos. I can shoot up to ISO 4000+ in many cases and recover just fine. Sometimes I push the envelope and will go for 8000 ISO, but I have to run through Topaz Denoise for that.
You are correct though the reason some of the bodies get a lower battery rating is because the EVF is a viewfinder that stays on when your camera stays on, therefore it is using power. I recommend using a battery grip, you can fit 2 batteries in there at once and it will often-times switch over from #1 to #2 when the first one is drained.