Although I'm not a big Christie fan, I always enjoy a good puzzle-structured detective mystery, and Cards on the Table is one of Christie's best, in my experience. There's a fascinating puzzle within a puzzle: Four professional and amateur detectives are invited to a party with four people who seem to be unconnected to each other or to the world of crime; but host Shaitana has given Poirot the necessary hint - Each of the four non-detectives may have committed murder in the past and gotten away with it. Then, before the party ends, the host himself is murdered. To solve that murder, of course, the detectives must also solve the murders in the past the four suspects may have successfully committed without suspicion.
This doubling of the puzzle gets lost in the TV version - although we do find out that the four suspects were involved in suspicious deaths in the past, these discoveries seem revealed incidentally. Thus the investigation seems to wobble around, and interviews that are clearly connected in the novel, progress in a somewhat hap-hazard manner here.
The final third of the TV version rewrites the book in pointless and annoying ways. I am not a homophobe, and I note that the Murdoch Mysteries program has dealt with the issue of homosexuality in a sensitive manner in several important episodes. But such sensitivity is lost here, because the original story had no room for the topic, one way or other; so the lesbian/gay characters are rather forced into their roles.
Inventing familial relations between suspects not in the original book, is also a bad move; it subverts the initial set-up of having four separate suspects to investigate. It also subverts the book's sensitivity concerning how the older female suspect (much older in the book) and the younger female suspect relate to each other.
Changing an important attempted murder scene - and thus the resolution of one of the older murders - might have been successfully pulled off, if there were a reasonable rationale for doing so, but there wasn't.
All these alterations lead to an unsatisfying denouement, leaving Poirot to expound more than he needed to in the book - including exposition of facts that had not been hinted at by any clues beforehand - a dreadful detective mystery faux pas.
Finally, the TV version mishandles the ethical themes of the book. Agatha Christie always hints at sympathies for her murderers, but in classic mystery fashion, justice must be served. Three of the suspects do end up punished, however indirectly, for their original murders (the 4th is revealed as an accident). In this version, one is brought to justice, one is exonerated, and the third reconciles with a lost daughter - and drives away - Huh? I really don't know what they were thinking when they produced this. (The Brett Sherlock Holmes series also lost its way towards the end, by rewriting stories to seem arty and up-to-date.) When will these people learn that admirers of the original stories want solid dramatization of the stories as-they-are, not some clever academic's Lit. Crit. re-imaginings?