Unless you've avoided all engineering I don't think the Guardian power plant and distributor offer anything.
FWIW, I seem to recall that if you fit a Guardian PP and PDist together there's
some kind of extra benefit... although I doubt it'll compete with a normal PP/PDist engineered exactly how you want them.
Main reason I unlocked the Guardian PP/PDist was that, back in the day, there was a very poor selection of modules in Colonia but you
could buy Guardian stuff there, so unlocking the Guardian modules meant you could build a half-decent ship
and not have to worry about wasting engineering mat's on a C-rated PDist or whatever.
On a related note, I suppose, the Guardian modules
can sometimes be useful for getting a half-finished ship to fly if you need to engineer an overcharged PP.
Course, these days, with pinned blueprints available, that's not likely to be much of an issue.
Personally, I'd say the Guardian shield-booster is almost as much of a must-have as the FSD booster.
For combat ships, obviously, every extra bit of shield helps but they can also be handy by allowing you to fit a smaller shield in, say, a cargo ship and then filling up the mil' slots with GSBs to compensate.