The only difference between M+AutoISO and fully manual is that with fully manual you have to keep an eye on the camera's meter scale to keep the exposure right. If you want a neutral exposure you keep the highlighted segment central, if you want + or - EC you keep the correct number of highlighted segments lit above or below the centre (which will involve counting). With fully manual your attention has to be kept on the meter scale, whereas with M+AutoISO the camera does the "looking" for you, leaving you to concentrate on other more important things.

If you want EC while in M+AutoISO, dial it in at the start or if it needs changing on the hoof, a button plus adjustment wheel is all that's needed (for most cameras). Set it then forget it - no constant distractions involved.

When using M+AutoISO you don't have to target base ISO - you can float it a few steps up from that (0.3 or 0.5 stops per step) so if the brightness changes suddenly you don't risk the ISO bottoming out. And despite the fact that ISO is floating, you still control it with your choice of shutter speed (and/or EC).