Mines a 2009, and whilst it was ok there were a number of spots appearing and i was shocked by it. Back was much worse. I couldn't get under tray fully off but wired off and krusted, hammerited then waxoyled it all. My indie treats sub frames as part of the service. Thankfully front bit I couldn't get to was fine but it was treated regardless. He said the same as Ned, it's a real problem unless early intervention and maintaining is carried out.
If it's been repaired , I personally would clean it thoroughly , give it two coats of BILTHAMBER 80 about 30 minutes apart then Hammerite the next day . Keeping up with further chipping off is simple , as opposed to powder coating having a hard and total seal which if pierced cannot be touched up.
My race chassis is powder coated but I have a regular problem with chipping off plus it actually lifting in sheets in places , so have to scrub right back each side of the peeling coat until it doesn't come off -- then Bilthamber/Hammerite --- it's likely that my chassis wasn't totally clean before coated but you'll never know until it peels off.
Echo what's been said above, powder coating is not a good answer for steel components that are prone to rusting and in wear areas such as this. As Rob said once the powder coating is chipped, that's it, protection gone and the rust starts in again (and is potentially more problematic as it will spread from the inside now)
Cleaned up cavity sprayed with Dinitrol and outside sprayed with a good underseal (which should withstand the stuff that gets thrown at it) would be my preference.
This is disappointing. My old 15yr old FIAT bravo front subframe appears to be almost identical to that on my 2007 147. The Bravo subframe is fine but my 147 subframe is really quite rusty in comparison and a 1999 FIAT is hardly a rust prevention benchmark. It is not the only area I have noticed that Alfa appear to have been cost cutting on under the car. It is difficult to work out what their priorities are sometimes. A car of the quality of the 159 should not be suffering this issue now or for some years to come.