I bought it because my choices were limited. I'm an avid module user. Railroading isn't a bad thing. But when it consumes the entire life of a PC, holding them hostage throughout the campaign. That's when it gets to be an issue. It would have been much better if the two parts were broken up into 5 pieces each with their own plot. And let the GM and the PCs decide when to stop. Tyranny of Dragons has the issue from day 1 where if the PCs don't buy in, then the world dies.
My biggest problem with Tyranny of Dragons/Rise of Tiamat is the puniness of the threat and the threat response. It's basically Informed Deadliness. You are
told that Tiamat is a threat to the whole world, and there are even factions of good dragons and Red Wizards and such who make noises about getting involved, and there's even a big battle at the end where dozens to hundreds good dragons are supposedly fighting the forces of the dragon cult... but if any of those forces were actually onscreen for the climactic battle, they would trivialize the encounter. In all seriousness, the ideal strategic response would be, "Instead of dividing your forces, take all the anti-Tiamat forces to the Well of Dragons, and when Tiamat comes through the portal, kill her." It turns out that nothing much that the other anti-Tiamat forces do matters anyway, at most they will reduce 75 HP from her total and weaken her legendary features some, which they could do just as well by actually showing up at the battle and simply hitting her for 75 HP of damage. Tiamat does deserve her CR 30 rating (unlike the Tarrasque), but she is still very killable, and if the 15th level PCs have even a couple of adult dragons on their side to act as meat shields/grapplers while they fill her full of arrows, even her full CR 30 form goes from "deadly threat" to "straightforward fight." To say nothing of what would happen if they have actual high-level NPCs on their side as well (Szass Tam's wizards, Harper agents).
So, from a simulationist perspective, I find the adventure path lacking mostly because I can't take the plot seriously. A world-shaking threat which is fought primarily by only four heroes is only credible if either 1.) it comes as a surprise, so that other heroes cannot join the fight in time, or 2.) there are no other heroes. Meaning that you're in a sword-and-sorcery world where all the powerful guys are bad guys (who don't care about opposing the new threat because it only threatens the good guys), or you're in a monster-dominated world where all the powerful guys are not (demi)human at all.
I could rewrite it to take place in a world where there are no friendly NPCs over 5th level, and it would probably work, but that would require such extensive rewriting that I might as well throw it all out and start over. For one thing, you'd either have to set it somewhere besides Faerun or else scrap most of Faerun's history. I think you could make it work in Ravenloft though: a band of dragon-cultists are trying to summon Tiamat into their domain, which would turn it from a relatively-benign-if-creepy place into a blasted wasteland terrorized by Tiamat.