Hello Ryan,
to give you a straight answer, all historic U73 units as they come from e-bay or vintage dealers are in desperate need of rebuilding, if somebody tells you it is already rebuild, please make sure who did it, and to what adjusting specs, pre distortion, etc.
There were several different version, the IRT based 6 or 12dB threshold, the TELDEC limiters, the EMI/Carl Lindstroem mastering units and the Deutsche Gramophone version. Doesn't matter what the front says, 73/a/b/TS, etc. it depends on the internal set up. Some units do not even share the same x-former...
Biggest problems that all controls are inside the unit, and if set up once, not easy to change.
I do not know if the U73b, if set up right, has a place in CD mastering, but for Vinyl the 73 were the standard in any european pressing plant up to 1980... It is quite hard to get the historic proper set up for the different types of lathe, and not to many people are around that can do that type of work, but if you get it done you are ready to go...
Seeing it from a technical point of view, the entire U series concept (check out the postings at Fletchers) is still valid today, biggest problem is to find good quality remote cutoff tubes, there are a few more E99F and EF85 out there than the 6386 you need for a Fairchild, but the adjusting and general maintenance is quite demanding. The loss of bass, and the drifting of your units makes my point.
Biggest payoff is the super fast response, and very stable frequency vs. compression ratio.
Regarding mastering in general, up to 1980 it was a objective issue, given strict rules "how to" by RIAA and DIN or other agencies, since then the "Mastering Part" became a more subjective or creative issue, several (record) companies started to stray off the given rules, to get better/more noticeable presence in an overcrowded recording/media market, so I do not know what to say about your home master issue.
Maybe some mastering guys can give some insides.... Pro and contras...