I wouldn’t call it cantankerous. I would say that it is commonly neglected, to the detriment of the car and the owner’s wallet.
The AWD system is simple a front differential sends power to a rear driveshaft. The rear differential sends power to both rear wheels. 2001-2002 P2 used a viscous clutch between the driveshaft and rear differential, and you could burn it out if you ran tires with different tread depth. No longer a replaceable part. I’ve been diligent with tire replacements on ours, and the viscous coupling still works.
2003-2007 used a Haldex clutch - sensors detect the slip, then the Differential Electronic Module (DEM) runs a tiny pump in the Haldex unit, that engages the clutch. The Haldex unit has a filter and fluid that can be changed. The DEM can be damaged by water intrusion, but parts are still available. It’s a reliable system, and still used on current cars. Other manufacturers use the system. You can replace the DEM, or pump, as needed. My R has the Haldex, the DEM and pump are original, and it has new fluid and filter, along with an aftermarket DEM protection plate.
The front differential, known as an “angle gear” was a poorly maintained part. It takes 700 ml of 75w90 gear oil. No specified fluid change interval. The seals can wear out, and fluid then leaks, leading to unit failure. Checking them once in a while prevents that. I have removed, and re-sealed, the angle gears on all the Volvos I own. I took the extra step of adding a magnetic drain plug when doing the work. I threaded the case on the XC, and when I did it on the R, I used a TimeSert for threads, just for the durability of having steel threads. I wrote about it in this forum years ago.
https://www.volvoxc.com/forums/showthread.php?29700-Angle-Gear-sealing-and-modification/page2
Our 2002 XC still has the original angle gear. The AWD still works. The car has 300,000 miles. Not really cantankerous if you pay attention to the system, but paying attention to anything on a car is not common among most Volvo owners, who tend to be non-car people.