I sometimes wonder if MOT testers sometimes record advisories on older cars just to appease VOSA, when in fact there wasn't anything specifically amiss, but just so that it appears they have done a thorough test (even though they actually have).
I take a couple of classic Triumphs for their annual MOT and they always record an advisory as 'underside covered front to rear with underseal'.
They both have the original dealer applied underseal, which isn't particularly thick, and any 'dodgy' welding would be quite obvious. In truth, neither have ever been welded. Whether they think that regularly passing cars of this vintage with no advisories would raise an eyebrow within VOSA circles, I'm not sure.
I have previously had advisories commenting about panels that are covered, such as cills on the Omega, or full length underbody shields, such as on certain Audi's.
I think a lot of it is just 'arse covering' with older cars. Unless the corrosion is quite obvious, then I wouldn't worry about their comments unduly. I wouldn't think there are many Omegas out there without surface rust on the front and rear subframe areas, which some MOT testers may warrant an advisory.