IMHO, it is way more work to emulate m68k Mac in QEMU. This port work is missing in current QEMU:
port mac emulated hardware: scsi, timer, serial, video, ethernet and etc
It is a daunting task to port features from BII to QEMU. But the upside is that you get patch and update from QEMU community.
Updating BII is relatively less work. What BII is not quite working now:
a portable JIT without object file hack and compiler constraint
a portable and easy-to-use emulated network.
For JIT, I'm still researching. I believe the current JIT in BII is port from QEMU old emulation method (I may be wrong about this). TBH, most modern machines are fast enough that I can't tell the difference w/wo JIT. It is not necessary to enable JIT. But to me it is a fun project to figure it out how JIT works in BII.
In terms of portable emulated network, BII has provided way and way too many networking options without proper documentation. Some like tun/tap are difficult to set it up and some like slirp are broken in 64bit. I tried my best to patch and fix sheep_net module for latest Linux and document it. It works more stable and better than it used to be. However, sheep_net is bound to Linux. Personally, I prefer Linux as development platform. So I haven't gone that far to find a better solution for networking.