CVTs and optimizing for the unskilled
I had a chance to drive two nearly identical Honda Fits a few years ago. They were 2014-2015s The only real difference was that one had a 6-speed manual transmission, while the other had a CVT. The comparison was interesting.
On the highway, the CVT won hands down, but this probably is not necessarily due to the CVT per se. The Fit has really low gearing in its manual transmission. (and was apparently something of an enthusiast vehicle.) Even in 6th gear, you're into the 3-4k RPM range if you're going 70 MPH. In principle you could have just put a taller 6th gear in for the highway and you'd have had much better outcomes. I'm sure some drivers would have struggled with needing to downshift so often, though.
Interestingly, around town it was actually possible to get significantly better MPG in the 6-speed manual. According to how it's measured in the dash, I was able to get 52 MPG average for one road trip in the 6-speed manual. I was doing what I'd call "light" hyper-miling. Doing my best to mind acceleration on hills, trying to coast to a stop when I can, etc. Even when I was in much less ideal conditions, I could easily pull 42-45 MPG out of that 6-speed manual under normal conditions. (and, assuming I didn't need to drive on the highway, that would drop me down to 36 MPG at best.) Part of what made this feasible was the extraordinarily low gearing in the 6-speed manual. I could comfortably drive in 25-30 MPG zones in 4th gear without seriously lugging in the engine.
The CVT really couldn't come close to the MPG numbers you could pull out of the 6-speed manual, but it was much more consistent. Highway MPG was more or less the same MPG as you saw around town, and as the driver, you had a lot less control over the MPG you were able to pull out of the car.
To put it simply, you could quite easily get much better mileage out of the 6-speed than the CVT, but there was a much higher skill requirement for doing so. (as an aside, broadly speaking modern automatic transmissions are more efficient than manuals, which was not true in the past. This is mostly due to technological advances in automatic transmissions. Almost nothing has an old-fashioned "slush box" any longer, and even those vehicles which do still have a mechanism for a direct connection with the engine to prevent losses)
That had been a fun experiment, and I hadn't really thought about it for years. But, my wife was driving us home from a party last night (I'd had a couple of drinks) in her relatively old-fashioned 2012 Rav4. It's got a 5-speed automatic transmission, and she only manages to pull about 22 MPG out of that thing. It's rated at 19 city, 27 highway, so she's nearly getting the city mileage all the time. What she's struggling with mileage-wise is pretty basic: she cannot keep a constant speed. She'll slow down, and then accelerate hard enough to drop a gear, get to speed, slow down, and accelerate again. Over and over, the time she's on the road. The poor transmission is shifting at least once a minute and often much more.
It was finally on that drive when I realized why CVTs make sense. If she were in a CVT it wouldn't have "dropped" a gear in the same way. CVTs obviously don't drop gears in any case, but if you push the pedal harder they will give you more RPM. But, they're a lot less sensitive to this effect than a 5-speed auto. So a driver with very poor pedal control will get very similar MPG out of a CVT as compared to a driver with excellent pedal control. And this is one of the reasons CVTs make such a big difference, the center of the bell curve of drivers will do much better in them. Many of the things we're sort of saddled with these days comes from some sort of attempt to optimize for most people. As frustrating as this can be, it's probably the greater good.