The MMU works fine, it's just poorly-documented and turns out to implement the incompatible draft-1.9.1 spec not the ratified and standard-going-forward 1.10 spec.
This is not Linux-capable though. The choices are FreeRTOS and bare metal. The chip is pretty unbelievable at that price though. You get a lot of functionality for $9. And it works - I have it on my desk.
Yes, it's the beauty of RISC-V: eventually, it's going to cover all the niches. From RFID energy-gathering chips up to desktop / datacenter CPUs.
Right now, its use is still very fragmentary, but Kendryte K210 chip shows that Chinese chip designers already got some real experience with RISC-V, and it's safe to bet it will be more chips with RISC-V coming from China every year.
At some level... does it matter? The BCM2837 (used in the RPi 3), is 40nm if https://www.raspberrypi.org/forums/viewtopic.php?t=137991 is to be believed; more advanced fabbing is good if you wanna go head-to-head with cutting edge ARM or even x86, but for a lot of stuff you don't need to be that good.
Alibaba have presented their Xuantie910 RISC-V processor, one month ago at Hot Chips.
It is by far the RISC-V CPU with the highest performance ever designed.
Nevertheless, to reach that performance, they had to add a custom instruction set extension with indexed addressing modes, as a workaround for the greatest defect of the RISC-V ISA, its incomplete set of addressing modes.