Take a look at their press release here: http://www.micromagic.com/news/CoreMark_PressRelease.pdf
They claim 11'000 CoreMarks at 4.25 GHz in what is presumably their more efficient running mode (0.8v, consuming 200 mW). That gives you 2.58 CoreMark/MHz which is quite poor even comparing against high-performance embedded, let alone the likes of the M1 (The arm M4 can achieve 3.42 CoreMark/MHz for instance). That indicates a microarchitecture that will choke on more complex workloads.
I think what they've done is produce a super simple 64-bit RISC-V core they've focused on clocking as fast as possible and pushing the power down as much as possible. Does a good job on CoreMark/Watt numbers (assuming they're even correct!) but useless for any real application.
Either you want a desktop/server style workload in which case their IPC, and hence total performance and efficiency, will drop through the floor, or you have something more CoreMark style in which case you don't need it running at anything like 5 GHz and you're far better off with one of the many embedded cores on the market.
Is the focus on efficiency covering up that it can't scale up to be actually as fast as the m1?
It all seems a bit too good to be true, what's the catch?