In my experience, Mannol is an excellent oil. Sure, it's run into some 'approval' discrepancies, but that's probably partly due to admin issues, amongst other things. If the oil was substandard to the point of being a risk to engines, it would be banned by the appropriate authorities (or at least the offending product would). Group IV (PAO) or Group V (esters) are not contained in all synthetic oils and, although they are beneficial in some applications, are not necessary for all. In fact PAO can cause some problems in engines such as possible seal shrinkage. To combat this, esters swell seals, so the correct balance is critical.
I've been using Mannol 7715 (504/507) in my 2008 Skoda Octavia 1.9Tdi PD BXE engine for 120,000 miles with absolutely no issues (a few of those years pulling caravans and heavy trailers). Turbo is the original, and that is completely dependent on good oil, otherwise it can self-destruct. I change the oil/filter every 10-12,000 miles (around a year). Prior to that, on a Mk1 Octy, I covered 100,000 miles with the same oil and same change interval - no issues.
I do think that, as oil and engine technology has become more complicated, people are becoming more paranoid on getting things 'right'. A few years ago, the average Joe wouldn't be thinking of PAOs and esters, and probably wouldn't even know what they are. Sure, putting the right oil in a modern car is the right thing to do, but making sure these oils are what they say they are is down to authorities such as the VLS and Trading Standards. Mannol has been investigated by VLS over certain claims, but whether this is simply down to making sure the correct paperwork has been submitted, rather than a problem with their oil, is open to debate.
All I can say is this oil has been tried and tested, by myself, over many years (even the years when there was dispute over its 'approval' claims). Having never had a problem with it, it's proof to me that it is safe to use with my car. The Mk2 has now covered 180,000 miles, and is still flawless in its engine performance.