I think there is a problem with the FreeBSD installer which can only be sorted out by reworking its opening stages.
For MBR users, FreeBSD installer enforces a mandatory slice which has to be nested containing at least one partition (for /).
This is wrong. Let's say the user wants to install FreeBSD on ada0s3.
The user should instead be getting prompted :
a) Do you want to set up a FreeBSD slice in the chosen primary partition ? Or,
b) Do you want to the entire chosen primary partition to serve as / ?
In case of a), the installer should work as it currently does already - setting up / (UFS) on ada0s3a.
In case of b), the installer should set up / (UFS) on ada0s3 (not ada0s3a).
In case of b), if the user needs any extra filesystems, those can be found in the EBR, just as with Linux. Generally, FreeBSD should move away from the BSD nesting schema in the coming times, offering it only as an option. I would like UFS to work under MBR just as it does under GPT - unnested.