[MK] Despite reading around in the forums a bit, it's the first time I hear the term underdog AI. I don't think this is related to specific tweaks that we make to the AI, but the the tatical settings these teams use. Also it might be related to your playing style...maybe expand on what you exactly mean by some teams having a stifling defense and list what formation and tatical settings they play with.
Without any offence intended, can I ask if you played Fifa09 MM much yourself? Beyond seeing what features are there and such? Because anyone who has played MM any length of time cannot fail to have noticed (and been massively frustrated by) the unbalanced underdog AI logic programmed into the game, basically let's consider two different matchups:
Example my league team is Portsmouth (before the leaking of all their good players), good team, rated 4.5* IIRC in Fifa09?
We play a home match against Liverpool, 5* team, lots of talent, it's a very tough match, Liverpool have skillful players, they pass the ball well and move well and perform realistically to their attributes of being world class players, I score early, manage to hold on for the win, they out possessed me and out shot me but I scrape a 1-0 win.
Next match is a league cup match, against League2 Lincoln City, they're a half * team, their best player is 62 overall and most players are mid 50s, the match kicks off, they constantly harass me for 90 minutes, despite their lowly stats they manage to have the stamina of a Gerrard, the tackling ability of Terry (this is every player mind, the forwards as well), the dribbling skills of Cronaldo (despite me having highly rated defenders of 85+ overall their 55 rate forward manages to dribble all around me), and their players seem to have taken their Drogba cereal and have the shooting ability of world class players, I end up losing and being completely frustrated playing against 11 super charged players who's stats mean absolutely nothing.
That is what the underdog AI logic is like and it's always there to varying degrees when you play against the AI (far worse at World Class/Legendary but still frustratingly there even at Professional).
Now I understand that the "magic of the cup" means that lower teams can raise their game a bit, and higher teams might get a little complacent, but a 50 overall defender cannot become John Terry for one match, a 50 overall forward cannot become Fernando Torres for one match, they can work really hard, they can use sensible tactics of getting men behind the ball as much as possible, marking 2 to 1 and such, but they should not get magical abilities to cheat their way to a win, that's jut bad, annoying design there.
So in short

that's what underdog AI logic is about and why we hate it.
Play against top teams and it's fair, if you lose it's because their abilities and play won them the match, play against lesser teams and you're battling their magical stat boosting beans and that's not fair gameplay.
So the question is, will this be fixed for Fifa10? Will underdog teams play fair? IE they'll try really hard, they'll use spoiling tactics as much as possible, but they won't get magical stat boosts?