I've worked with designers with limited JS knowledge on a JSX-free react code. There may have been a short learning curve, but it didn't seem to slow them down much at all, even really early on, certainly not after a week (at that point our codebase was coffeescript, so JSX wasn't a great option).
It may have helped that we converted an existing code base (so there was a lot of html-as-JS example material to see), and devs were around the corner for any questions - but in my experience, JSX did not appear to be relevant, not at all.
The only advantage jsx does have IME is that it enables copy-pasting real html (sometimes with minor alterations) into your source, and that's a mundane but constant minor boon. All the examples on the net are in html; and if e.g. you use browser dev tools to rearrange a page to look like you want it you can easily copy-paste that back into the source. I don't think that advantage is worth putting up with JSX for, but it's not irrelevant either.
That cuts both ways, though, since that easy “copy and paste” turns into “copy and paste and find everything which conflicts”. That's annoying and in some cases even a source of confusing bugs.
It may have helped that we converted an existing code base (so there was a lot of html-as-JS example material to see), and devs were around the corner for any questions - but in my experience, JSX did not appear to be relevant, not at all.
The only advantage jsx does have IME is that it enables copy-pasting real html (sometimes with minor alterations) into your source, and that's a mundane but constant minor boon. All the examples on the net are in html; and if e.g. you use browser dev tools to rearrange a page to look like you want it you can easily copy-paste that back into the source. I don't think that advantage is worth putting up with JSX for, but it's not irrelevant either.