I personally don’t ship it. The 10+ years’ worth of an age gap would be problematic for some people on its own, but for me the thing that makes me not want to ship it romantically is the fact that Nick effectively rescued Trubel, kept her from being arrested for murder, and that sense of gratitude makes for a bigger power imbalance than the age gap alone, which I’m uncomfortable with.
I think to some extent, it depends on the perspective of the shipper. I’m right between Nick’s age and Trubel’s, so I would feel weird shipping a girl barely old enough to be out of college with a guy my age, much less someone who had a few years on me. And I think fans of any age have a responsibility to consider the ramifications of age and power dynamics in what they ship, the fanworks they produce, and how they interact with younger fans.
That being said, for a fan who’s Trubel’s age and shipping Trubel with Nick, I understand it a little better. A lot of young people find themselves attracted to people quite a bit older than they are (see my massive crushes on celebrities from Jensen Ackles to Tom Cavanagh, the latter of which is older than my own father). And having a place like fanfiction to explore that safely is, to my mind, a good thing.
Where my worries start is with the notion of the writers making anything romantic between Trubel and Nick canon. That’s a context I’m not comfortable with.