The gameplay is self-explanatory so there's nothing really difficult about these games, but when it all comes down to, It's all about leveling up and nothing more.
For example: In HeartGold I was ready to take on the E4 champion without any Pokemon taking serious damage from the previous guys, I managed to take down his first Dragonite and to severely damage his second one, he switches Pokemon and kicks my ass.
So I'm back outside and I decide to level up my team, eventually I reach a point where the Pokemon aren't getting enough experience, so now comes the tedious part, defeating wild pokemon and getting small amounts of experience for ALL six of 'em, if only one of them is weaker than the rest then I don't stand a chance because the final battle is all about luck.
At this point the game gets even more boring than it already was and I just don't play it anymore.
[spoiler]NOTE: I have defeated the champion and what do I get? a new region to waste more time on? ... no thanks.[/spoiler]