There were things that didn't make any sense in the Kung Fu Panda 2
*It does not make any sense why Shen was evil, it show first he would make weapons with fireworks gunpower because he wanted to conqure China. But how can he become evil by being interested to make weapons with fireworks gunpowder and why he wanted to conqure China that had nothing to do with him and not only his own kingdom Gongmen City. Shen would at least get his will to rule over Gongmen City since he was at least a prince and the only heir after his parents to rule over Gongmen City, usually princes and princesses are power hungry because of jealousy if they have an older siblings who are the heirs to the thrones and will only rule over their own kingdom instead of entire country or world that have nothing to with them.
*Why would Shen make an end of kung fu when it had nothing to do with his backstory? Shen's main objective was to conqured China and he was even a kung fu warrior.
*Shen created a cannon that would make an end of kung fu, but it's not only kung fu it should make an end of if there is anything else that are as powerful or more powerful than kung fu. Are not kung fu warriors smarter, too quick and has high strength to destroy the cannons? Tai Lung would likely be capable of handling attacks from Shen, Tai Lung was shown to be cunning and swift enough to avoid firing weapons and was able to defeat the Anvil of Heaven, the Furious Five and Shifu on his own. Shen must at least have kung fu skills and learn his wolf army kung fu to defeat kung fu warriors and could study and develope a dark and stronger kung fu style to make an and of other kung fu styles. Kai and Taotie could make an end of kung fu, Kai easily defeat kung fu warriors by taking their chi on his own and once Taotie created a machin that easily defeat Po, Shifu and Furiouse Five (without Tigress).
*Why did Po and the Furiouse Five only tried to destroy Shen's cannon first, didn't they know Shen had more than one cannon and could create multiple cannons quickly that they should rather stop him from doing instead?
*Wasn't most important to stop Shen from conqured China than to stop him to make a end of kung fu.
*In Kung Fu Panda 2 Shifu mentioned Po found inner peace at such a young, in Kung Fu Panda 3 Mr. Ping mentioned he raised Po for 20 years before Mr. Ping told him he was adopted and behind-the-scenes of Kung Fu Panda 3 it's mentioned that he is in his 20s and in Secrets of the Scroll Po was a teenager 10 years ago. But Shen waited 30 years to rule over Gongmen City alongside his cannon probably waited ever since then he started to use fireworks as an weapon and it's not even mentioned or shown that Shen began to use fireworks as a weapon years before or it was 20 years ago he attacked the panda village where Po lived when he was an baby and how how could Shen's parents be concerned for a long 10 years before they had to know Soothsayer's prophecy?
*Why did Thundering Rhino let himself get hit by cannon instead of reversing aside?