Read 100s of cases on Canlii.org. It's a public database of all appeal cases, and believe me, I've read tons of these cases. The VOs always express concern with age differences, but they are very suspicious of people who don't speak a common language. Also, typically near the end of a case, they always ask about the couple's plans should the PR application fail, and the VOs always express concern if both parties do not have a plan, or are only set on residing in Canada, which seems to be what the case is here.
I've read so many Canlii cases, but I especially read the ones about Vietnamese spouses, because my wife is Vietnamese. I read all the refusal cases on Vietnamese, and I read things that I didn't even realize were red flags. For example, I didn't know they expect you to do every single thing related to a traditional wedding, such as street procession, exchange of gifts, praying at altar of grandparents, because I'm not knowledgable with Vietnamese weddings. Before our wedding, I asked my wife if our wedding is going to have all this stuff, and she says yes, so I was relieved because I know our wedding will have been performed to the VO's wishes.
I read lots of Vietnamese cases where the VO expressed concern that the couple had a traditional wedding, but for example, didn't hold a street procession because they thought that meant the couple didn't want their relationship to be public.
Canlii.org is a huge resource and its very helpful for people trying to see what goes on in the eyes of a VO