If I have to remove the rear wheel to change a flat, the number one issue I have is the awkward bolt on my older Nuvinci hub. The number two issue is getting the wheel out around the chain tensioner (I have a rear-derailler-style tensioner to make my hub compatible with my vertical dropouts and my two front chainrings). Most IGHs don't have my weird bolt issue, so if I had to choose between a bolt-on IGH and a quick-release derailler, based entirely of ease of tire changing, I think I'd still rather go IGH. I feel like wrestling the wheel around the derailler pulleys is at least as obnoxious as having a bolt-on wheel. But that's just my opinion. In reality, I don't mind at all having an extra tool in my emergency kit, and beyond that extra tool, the only real issue I have with changing a flat is to make sure I'm able to do it. The extra time that may or may not be involved to remove a bolt never really figures in given that I go for months at a time without a flat. That might be different if I were getting flats more often or if I routinely had to remove the tires to squeeze my bike into a car. Although I'd probably change my tires before I looked at a whole new drivetrain to address frequent flats. And I've seen a few options that allow for a quick release with a rear hub, if that's a big issue for someone.

As for shifting under load, some hubs handle it and some do not. I know my Nuvinci sort of shifts under load. It's harder to shift, but theoretically possible. I worry about breaking a cable, so what I do is apply moderate pressure to my grip shifter, and when my pedal rotation hits a dead spot, the shifter moves. The newer Nuvinci is made to shift easier under load than at a standstill, as I understand it. So if you like the idea of an IGH, but also want to shift while under load, there are still options.