The 4 GB barrier isn't an issue with SCSI controllers.
Yes, it is. The same limits apply to both, SCSI and IDE.
You need a SCSI driver and a file system which both support the same set of commands for bit hard drives.
If you use the original FastFileSystem, for example, it does not work with SCSI, either. Even the new version which comes with OS3.5/3.9 does not work with most SCSI controllers. You have to use SFS or PFS or patch the SCSI driver to support NSD commands.
The only difference is that SCSI controllers usually support SCSI commands without bugs, so you can use file systems which support Direct-SCSI. And the SCSI controllers from Phase5/DCE also support TD64, so you can use file systems using this. For example PFS3 / PFS3ds and FFSTD64.
Bye,
Thomas