Honestly I was impressed how different their entire design and playstyle is. It’s like taking core Euro civ and comparing to Lakota, in AoE III:DE.
And it’s even impossible to compare to changes between civs in AoEII:DE. Just now after decades we got some changes for common, core buildings (Polish folwark replacing ‘mill’). If you learn one AoE2 civ you know pretty much other ones.
Playing as Mongols is very different than English when it comes to basic building strategy, even way how and where you collect food*…
*III also have these things but more minor and among a lot of other common, core systems. Like Japs not hunting animals but building shrines-houses to attract them and gather XP.