Flying in to contribute my 2cents!
I have the broadcom 94352 HMB in my laptop and I had the same issues. with macOS sierra 10.12 I had the option to enable Auto-Unlock with Apple Watch but it never worked. in mojave, many things were fixed and airdrop/handoff/imessage/etc work for me now, but Instant Hotspot still doesn't work and Auto-Unlock appears to be no longer supported in System Information > Wifi. The latter may be because I moved from the old Airport fakepciied method etc. to AirportBrcmFixup, which may more accurately reflect the true capabilities of the card.
From what i've read elsewhere, it seems that some special firmware opcode(?) is required for the unlock to work, and since the 94352 isn't native, it's firmware doesn't support this, whereas the 94360 does. what's the status of the NIC release though? does it actually work? the responses above are confusing. idk if they're using the native or the HMB