I know a lot of people who think they can touch type the number row, but I've yet to meet anyone who can actually do it at anything resembling their normal typing speed and without errors. If people could there would be no reason for the numberpad to exist. I'm a very fast typist and I've always struggled with touch typing the number row.
Thirty minutes into using a split 40% keyboard and I was using the number row at full speed, something that has eluded me my whole life. And of course, it's a hell of a lot easier to hold down a key with my thumb and touch type the qwerty row than to make the normal awkward stretch that fullsize keyboards use.
That told me pretty much everything I needed to know. Once I learn a layout I can type any key that's on the home row or the rows that are directly above or below it and a thumb modifier faster than I can type any keys outside of the three home rows sans a modifier.
The idea that many people have that somehow 40% keyboards are about having less keys at your immediate disposal is completely backwards. There are 36 keys that are within a 1 key displacement of the home row. With the natural 36 and two thumb modifiers on each side that's already 180 keys that are all touch typeable, and that's without even getting into any of the fancy stuff like being able to tap for one function and hold for another, or using two modifiers at once, or using a vim style leader key, or chording, or having modes that you can switch into.
The smaller keyboards aren't about having less keys, it's not like I woke up one morning and decided to never use function keys or curly brackets again, it's about removing all the cruft that that can't be pressed easily and mapping it in a way that it can.