This is a strange one, so I'm going to split it up.
Things I liked:
Monty's struggles of self-acceptance. He's an arrogant aristocrat, a drunkard and a rake on the surface, but there's a lot more going on and as we learn more about him, it's clear how he got to be so messed up. But he's got his best friend Percy and his sister Felicity, who are more aware of the world around them and help him see what he's always been so blind to. I did like seeing him grow up and learn new things about himself, and that it doesn't happen all at once in a giant ah-ha moment but a little at a time as the story progresses.
Percy was also great. I like that the author acknowledges people of color existed, and as more than just slaves. He was born in a high-class family, but being interracial and a bastard doesn't give him much standing. He's treated as second-class, and while Monty might not treat him that way or understand why anyone else would, Percy is aware of his position in society and how tenuous it is. And that's even before the reveal
that he has epilepsy and his family wants to put him in an asylum because they're tired of dealing with his fits.
Felicity, Monty's sister, knows her own mind and isn't afraid to use it. She wants to study but is limited by her sex. She also helps hold a mirror up to Monty's face, but she's not there just for the benefit of the male characters. She has her own agency and makes her own decisions.
As a road trip gone askew, this is a great book and not nearly as silly or whimsical as I thought it was going to be. And I like that it didn't always follow the tropes to a T, so that it kept you guessing in some places.
The things I didn't like:
As a historical book, this is somewhat lacking. There's nowhere near the level of details that I expect from a historical. Nothing is really described, like the author is expecting the reader to already know what all these places looked like back then and so doesn't have to bother setting the scene. Except for the lack of pay phones, the author could've easily placed this story in the 1960s or 1970s and not have had to change anything except some character names. The rather modernistic manners of the characters would have made a lot more sense and rang truer than they do placed in 1720-something.
The language is definitely too modern. Look, y'all, "abso-bloody-lutely" is annoying AF coming out of mouths from today's youth. It has no place coming out of these characters' mouths. They had their own slang in the 1700s. Use it! There were a few other modernisms like that too, and it just pulled me out of the book every single time. This is basically a historical for people who don't want to read historicals.
There were a few continuity errors too. At one point, Monty has to stop to put his boot back on. I went back several pages to see where the hell he took off his boot - he didn't. At another point, Felicity is hurt rather severely and it's several scenes before she's able to properly tend to her wound. In between, there's an encounter with some rather important people who I would expect to be far more observant than they are. There's no mention at all that Felicity is attempting to hide her wound, yet it's not mentioned and neither does it seem to even bother her. What the hell happened to Lockwood?
Then there's Monty's dad and everyone else practically having no concern whatever that Monty's got a liking for boys. Sure, the author does bother to point out a couple of times that sodomy was a big no-no and even bothers to mention some of the punishments that could befall someone because of it. But then everyone just acts like it's no big deal. Extremely distasteful, sure, but nothing you wouldn't bring up in casual conversation during a ball. It felt like the story and the characters were making far too light of something that could get you killed. The fact this is YA doesn't justify that, and this is far too much a trend in many an M/M historical. I was disappointed to see it happen here too.