I've not heard of any issues endemic to the "short inline six" (si6). The 3.2 (normally aspirated) si6 in my parent's '09 XC70 is still giving them trouble free service at close to 200K.
By 2012 that engine was in it's 4th or 5th model year so presumably most of the kinks had been worked out (at least those that could be fixed). If you wade through the "Manufacturer Communications" on safercar.gov you'll find a document or two about excessive oil consumption in early si6 engines. They made some design changes in 2010-2011 to help but it seems to have persisted in some cars.

To avoid that when making a purchase, one might ask pointed questions about oil consumption and misfire troubles and pay close attention to oil change records to ensure that they were done. (Same as one should for any "white block" Volvo.)

-Will
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XC60 / Odyssey