With regard to auditors comment. Clearly there wasn't an issue...

With regard to auditors comment. 
Clearly there wasn't an issue with the prior audits carried out by the same auditors so presumably the partner in charge has changed.
Another partner or audit firm could just as easily have made no comment & carried on as before so the distinction is one of a personal view of one individual rather than a failure to comply.
Mngt have stated that even if they had to change valuation they would still disclose the fair value result & use that as their key metric, similar to banks using cash earnings instead of reported.
The company has consistently used the same method for decade+ & generated more cash than they have invested in PCP so there is no issue with regard to the ultimate profitability only the timing of those profits.

So what is the actual consquence to shareholders of a change of methodology other than sp fluctuations?
IMO the biggest potential issue is that you can't pay out dividends without statutory earnings to pay them from, so a potential significant profit write back could restrict future dividend payments at least in the short term.

The issue of operational performance is relatively easily solved based on the prior company performance in the short term however the audit issue could have
a much longer impact.