My husband wrote a 3 page essay describing our relationship, and also at the end spoke about how I lived in India and how we overcame so many odds in our relationship because of that. I think he mentioned us raising our children in Canada to give them a better opportunity. When I read it it didn't come off as opportunistic, so I think it really depends on how it is written. If you spend the whole essay focusing on that then I can see how it can come off being suspicious.
I think the problem happens because the Immigration officers are suspicious of everyone, they are trained to be. But my husband and I have a genuine marriage. We have two kids for heaven's sake. So we know we have nothing to hide. However, I know it is such a stressful game to try and think from the mindset of these immigration officers. It can drive you crazy.
I personally think if your relationship is true and genuine it will come come across that way, just focus on your relationship and what you mean to each other.
I wrote a much shorter version of an essay...Mine was one page. I focused on our relationship and talked about our children.