As an example of the limitations being referred to, take a look at the match engine. It hasn't changed in donkey years and the AI has become so linear it's a joke. E.g. the AI actually cheats by scripting mistakes and poor performanes into your team/players to give the impression that the AI is countering your tactics like a real life manager would. The AI doesn't have the ability to form genuine counter tactics, or exploit weaknesses in your tactics or lineup - you just end up losing games because the game has decided you will. It's gotten to the stage where having good tactics and a well put-together team with great individuals means alot less than it used to in older versions of FM/C.
The match engine itself also has players in specific positions doing the same repetitive motions, no matter what your instructions or what their abilities, because those specific positions are scripted to play a certain way. For example, you'll never see a centreback dribbling forward past 5-6 players like Lucio or Beckhenbauer (or even big Sol Campbell at one time) used to, no matter how much you tell them to, so if you somehow managed to get a cultured and gifted attacking defender, those attacking abilities are useless within the game. Same thing applies to many other positions, e.g. fullbacks. The Cafu's and Roberto Carlos' in real life don't exist within the match engine as fullbacks only play one way. This is all because the game is stuck with a match engine from 5+ years ago that they've only incrementally updated, and won't really make major changes to.