Other content worth warning about: bigamy. Regarding the portrayal of mental illness, people with a fairly academic interest about historical and cultural perspectives of "madness" will probably find this book quite interesting, but I suspect that people who are personally affected may find it incredibly upsetting because

one of the characters has "latent insanity", and is essentially the villain of the story. Note also the 'forced institutionalisation' warning.

I also considered warning for homophobia or biphobia because

the main character is almost certainly in love with his (male) friend, and later falls in love with that friend's sister, who looks just like him. So both the main character and the book itself seem to carry some internalised homophobia or biphobia.

The main character, though likable (in my opinion - your mileage may vary), holds some views which are somewhat uncomfortable to modern audiences, especially about women. The author tries to give some sort of an explanation for his opinions about women, but it's a weak excuse at best and not really logically consistent with the timeline of when he has those thoughts. If you going to read this novel, I would say that you have to be prepared to be "preached to". I think that's a warning that can probably apply to most literature of the era (I think all that I have read lately have had at least one moment where the authorial voice gets a bit "preachy" about religious or moral matters, and that's to say nothing of the characters) but the moral preaching does seem particularly strong here.