I'd argue this isn't down to the game favoring underdog teams/tactics. It's the kind of dynamics that's to expected by tactics that aim to exploit whatever engine issue there is. You'll raise quickly with the most average/poorest of players never really managing nor understanding why, which if anything is what shouldn't happen. It causes so much frustration evidently. You can almost guarantee that most of the stuff getting shared this year won't work in the next coming iterations, as in the official forums there is already loads of players getting annoyed by central positioning that allows such to happen in FM 2017 (same as the wide "crossing issue" was criticized on 2016). You haven't gone through a natural rise then. As more teams keep it tighter, you are trying to unlock packed defenses with defenders that are better than your attacking players man by man. That's all a huge problem in so many ways, and sadly nothing a one size fits all tactic testing will ever solve.
You're then handicapped in two ways. Firstly, by your worse players. And then the virtue of that one-size-fits-all approach, just hitting continue, never applying simple logics that can help to stretch defenses by for instance stretching the pitch going wider. Quite the contrary, the engine holes targeted oft tend to be plugged a bit with defensive AI approaches. Additionally, if you go through the tactics, most of them have minimal to zero defensive cover, which influences further as the more defensive sides get, the more they lure that out. It's not going to happen as much as long as an opponent pushes aggressively up. That's then worse defenders being exposed for position against better attackers. Leicester couldn't rely on opposition to collapse when flooding the middle nor did they ever play with so little caution. Given up on exploits as that would need a perfect engine. I just hope that one day the game will include better feedback and assistants!