Although I've only had 2 omega's (both 2.5TD's) I have had countless other vaux's inc V6's, done loads of engine conversions etc, I think they have to be worst DIY engine vaux do! The BMW TD engine is far stronger and miles more reliable. Also a few simple modifications can take the power to that of a 2.5 V6 with loads more torque.
Hmmm, my experience is that V6 is way more reliable, and cheaper to fix when it goes wrong.
The BMW 2.5TD(s) is way overrated imho, not particularly economical, too heavy, prone to coolant silting up, noisey, and bloody expensive when something needs doing....
Personal experience I guess. My experience with the V6 are problems like Heads gaskets, coil packs, oil coolers, big ends, tappets and EGR that’s without all the sensor faults like lambda, CTS and AFM. The only common fault with the TD engine is the bottom pulleys leading to premature were on the shells but that’s only because people don’t check/tighten and replace the bolt! Also owning a Range Rover with the same engine I've never had problems with coolant silting or engine noise, infact for an old diesel engine they're remarkably quiet when warm.
HG failures are very rare on GM V6. DIS failures seem to be rare, unless they get wet. Oil coolers appear to be done to improper servicing. Big ends - only ever seen one V6 with big end knocking. Sensor wise, V6 very good, with exception of crank sensor - far better than BMW engine, where sensor failure is common, esp MAF (on the later ones that have MAF), only the ECU rarely lights the EML when sensors fail! Never seen a real MAF failure on V6 - 'MAF Failures' on V6 tend to be garages who do not know how to use the codes that the engine is telling you.
Rangies and BMW suffer from the silting, as well as the Omega. Whether or not it is a (lack of) servicing issue, I haven't yet worked out.
And don't get me started on bloody viscous fans!