*Possible slight spoilers*
So, first off, I've only heard of this film since it came out (it may be the first movie that I ever heard of Sir Ian Mckellan before he came into my purview more for roles for my era in XMen and LOTR). I never saw it before, though.
The film has some incredible acting (Ian McKellan is of note) which is already known because of the awards the film won. I think Brendan Fraser is slightly miscast here, not really being the type to embody the tough military guy type. That is, until we understand that it is all a facade for his character. I don't think Brendan Fraser is a good enough actor to have pulled that part of the performance off fully, but it makes the casting make more sense once the film brings this to light so I don't fully look down on this casting choice. Fortunately, Brendan Fraser has always been a charming actor (I think it's because he seems like such a good dude in real life- the guy is one of those that was just born to be a dad it seems) and this works once the film reveals his true character.
Unfortunately, if you are fascinated by the film, you'll quickly run to google to find out if it's a true story. Look, no Hollywood film is a "true story." Based on a true story is just that: literally only based on the fact that someone said this happened and then extrapolated into something to sell.
Even that is stretched thin, here, as it's only slightly based on a true story. Brendan Fraser's character is not based on anybody that existed which means the entire movie is completely false- which angered somebody for... no reason worth taking seriously. The only thing that is true is the facts around James Whale- which is enough. Everything else is made up based on some true things that really did happen but bringing forth conclusions not based on any evidence. It's reality fantasy, as all stories truly are at some point. Even the ones you tell your kids about who you were when you were their age.
With that out of the way, though, I find it a missed opportunity. If this is the case, why not make it unclear if Brendan Fraser's character is even real at all? It is a fact that James Whale suffered from strokes which caused him to have some form of dementia, or other nueral issues, late in life. Why not, instead of making up a fictional character, make that the driving force of the story? Why not have that be a mystery since there is so much mystery behind James Whale's death?
I think the choice to make Brendan Fraser's character fully realized, merely because it's the singular lie in the film, is the ultimate missed opportunity. The film should never have the scenes where Brendan Fraser is with other people without James Whale. He should not have a clear through line after James Whale's passing. It should be a mystery if he ever truly even existed in the world of the film (in real life he is not based on anybody purposefully).
Given how much of the film is true, and how much is not, to me it just makes more sense to play with the mystery of it. Have it be unclear so we can further the character study and have the audience as uncertain with reality as James Whale would have been at the time.
Anyway, Sir Ian McKellan's performance should not be understated. He gives an incredible performance that very much drives the entire film (and he is much funnier than Brendan Fraser in this although who knows- maybe it was mandated). However, I think the writing is more simple than it needed to be given the subject matter.