I know almost nothing about these, other than they can be used for S-parameter measurements of any combination of optical and electrical connections. i.e. O/O, E/O, O/E, E/E.
Also be aware that if used with a 3.5" floppy they store data using LIF format which makes data transfers a bit difficult. GPIB should be quite usable but finding software to work with them.. ??
I would expect other gotchas!