Do any UHS-III cards actually exist in the wild? I was searching quite recently because I was curious what they cost, and was unable to find a single product anywhere. (Note: There were some off-brand UHS-I cards with U3 speed that were falsely being sold as "UHS-III.")
I was able to find a single SD Express card for sale, and announcements for a few others that don't seem to actually be available yet, but are there any cameras or other devices with an SD Express slot onboard on the market yet?
This is just a guess, but based on claimed speeds it seems like only the highest-end UHS-II cards are capable of maxing out the half-duplex speed of that bus, so there would be little real-world benefit at this point of a CF-Express card, and since it operates at UHS-I speeds in backward-compatibility mode, you'd actually be buying a slower card that UHS-II if you had a UHS-II device. Or, in the case of the MBP, putting in a slower slot for the UHS-II cards that people are actually using today, in favor of a future standard that nobody uses yet.
The slot in the new MPB is UHS-II, which has full-duplex speeds of ~150MB/s, and half-duplex speeds of over 300MB/s if you put a UHS-II card in it, and if there's even a UHS-III card on the market, please do post a link.
These Transcend cards are only UHS-I, which operates at one third the speed of UHS-I (50MB/s full-duplex and 100MB/s half-duplex) so the limitation in speed isn't the slot--the MBP as-shipped supports a hypothetical UHS-II flush-fit card with up to 300MB/s read/write speeds. However, fast UHS-II cards are really expensive so Transcend presumably didn't want to triple the price (or more), when relatively slow speeds are probably sufficient for the main use case of this thing.
If you want a ~300MB/s SD card you can buy one right now and use it in your MBP, it just won't sit flush and will cost you on the order of $1/GB of storage (Lexar has a few that are cheaper, but they're also much slower in write speed). I also can't seem to find any 1TB UHS-II cards, period.
All that said, the one area that I wish Apple would be less dead-set on an integrated architecture is M.2 SSD slots, given how quickly storage advances, how fast M.2 SSDs are, and that flash storage has a finite lifespan. Even RAM I can accept for most machines, but given the bulk of the current MBP surely they could have squeezed an M.2 slot in, at least as a "second drive" storage in addition to the integrated storage, and it's really disappointing that the desktops don't have any way to upgrade fast internal storage. C'mon, just slap an M.2 slot in under that screw-bottom on the Mini!