I posted this somewhere else, and will probably get flamed for it. Anyway,
The definite flaw of Chrono Criss is the abundance of playable characters who have little to no character development and contribute nothing unique to the story. This alone also caused the developers to take out Guile's backstory as Magus, effectively making a bastardized character with no history and leaving Magus's history as a big question mark. Other than the character problem, the only other non-opinionated, legitimate criticism is the cramming of a lot of plot into some speeches on Opassa Beach. The plot itself is godlike and totally consistent with itself as Trigger's; the only unsolvable enigma in the game is the Dead Sea, whose mechanics aren't exactly explained (though fans have tried and succeeded with clues). This does not constitute a plot hole, along with other unexplained occurrences, as Trigger fanboys so vocally point out. The hard definition of a plot hole is something that actively contradicts something else in a plot; simply because something is unexplained does not mean it contradicts anything. It is entirely logical that the Kingdom of Guardia could fall, and Crono and his crew are die, considering Masato Kato wrote it (saying Crono & Marle were involved in some kind of incident) and we have no evidence to the contrary. Just because it doesn't appeal to fans doesn't mean it is not canon.
The Chrono Cross hate in a large part is blown out of proportion by a few vocal critics who rant loudly. This may sound like a "Silent Majority" copout, but considering that Cross outsold its expectations in the United States and generated a sizable profit, it is not. Chrono Trigger fanboys wanted to play as Crono again, have the same old battle system, etc. while the Chrono Cross developers explicitly stated in interviews that they wanted to innovate and explore more aspects of the Chrono mythos. This meant a new battle system, a new overworld (though in actuality its just a few square miles in the ocean of the Chrono Trigger overworld), new locations and characters, and a new cast. This is why they said Chrono Cross is not a direct sequel to Trigger in terms of gaming. However, in plot, Cross directly ties up loose ends of Chrono Trigger, and exists solely becuase of the actions of Crono in the first game. Cross never contradicts anything stated in the first either, an amazing achievement for a game with such a labyrinthine plot. Altogether, Cross seems to be like a masterpiece work of art compared to Trigger's action adventure movie; the scenes are pastoral, the characters vivid, the music perfect (can't think of a better word for the soundtrack), and the plot deep and organic. Cross received rave reviews from professional game reviewers, even receiving one of four tens ever on Gamespot. However, the myth that it "sucks" is still perpetrated by Trigger fanboys who cannot distinguish their own dislikes and expectations from critical fact. Cross is one of the best expressions of art in video game form ever made, though on some fanboys, it was entirely wasted because of narrow-minded expectations.