Cons:

The action doesn't pick up until 70% of the way through. This isn't unusual for Victorian era books, but none of the characters are interesting enough (or interesting consistently enough) to carry the story for that long.

This is one of the earliest examples of detective fiction, and you can really tell. It's before people figured out how to write mysteries that are actually fun to read. Like Poe's earlier detective fiction, it reads at times like math word problem where every hole is plugged and everything done and dusted once the story is over.

The "love interest" (Laura Fairlie) is so weak, literally and figuratively. The character isn't actively hateful, but dear god, the author's lack of interest in her really shows. She is despised by her husband, treated like disposable trash by her husband's friend, and is so utterly forgettable nobody can even remember what she looks like. She is a nonentity in every sense of the word, and also the paragon of Victorian womanhood and virtue.

The MC (Walter Hartwright) is so lame. It doesn't surprise me he was the only one in the book to find Laura irresistible. His endless ramblings over his love for her is cringe.

Pros:

Collins liked spicy women and it shows. Mrs. Catherick is fantastic. The book really became less dead when she came on the scene. Wish he had given her more words.

The last 30% is fast paced and highly readable.

Sir Percival Glyde was unexpectedly sympathetic, and "the secret" is not bad so far as that sort of thing goes. I actually figured he was some kind of impostor, but he's actually a victim too. The inheritance should have gone to him, but due to unfair laws of the time towards children born out of wedlock, he would have been legally disinherited.

In general, I like how the villains in the book are actually legitimate victims too. Laura's aunt the countess fosco too was deprived of her inheritance by her own brother (Laura's father) because he didn't approve of her marriage, which is not a crime but certainly a sin on Laura's father part.

All of which ties in with Collins' very progressive notions of social justice which I enjoy a lot in his books. He's never preachy like Dickens when it comes to social criticism.

Conclusion:

Modern critics are hostile to the book and for once I'm with the critics. While the last 30% is good, it's not quite good enough to justify the slog that was the first 70%.