Marth is far from underpowered, Falco is far from overpowered [in my opinion] - Marth generally has better zoning than Falco (Falco has back air sort of - his fair is meh and his nair and dair don't have much in the way of disjoints) while Marth has fair, nair, and side+B. Marth's mobility trick works best in the air while Falco's is significantly better on the ground. Falco is a bird so he can jump four times, but his jumps aren't that high and he falls way faster than Marth. Falco also has four jumps because his lateral recovery that doesn't leave him helpless [Falco Phantasm] drops him so far after using it before he can act you need all 3 jumps to get barely above the height you were at when you used Phantasm.
Not sure how helpful this is, but if you want some extra range, angle fsmash up and you'll slide forward (you can slide back if you angle it down - good if you want to tipper and they're too close, or if they're hiding in shield to increase the distance between you two) - this increases its effective range (if you uncharged fsmash angled up while walking you slide a long way). If you haven't been angling fsmash, you'll see (once you start doing so) why OHKO tipper would make Marth far and away the best character in the game (and hopelessly broken).
Marth also has a counter - this isn't a bad thing to keep in mind (still risky) and he no longer has RCO lag so getting off the ledge isn't so terrible. Falco is also (in some ways) Ken combo fodder - his fall speed is great for setting it up in my opinion.
The generally considered best character in the game is Lucario (or just was the best?), and ROB and Sheik are considered very strong as well.
You should play Gold_TSG's Marth - his movement with Marth is rather scary (I find I have to rely on reflector to outspace him and try to get combos going, while trying to limit him with blaster - unfortunately I'm not as good at it as I should be). Falco vs Marth may not be a terrible fun MU, but it's far from unwinnable, and Falco isn't broken in this MU (at least, I haven't seen/found how to shut down Marth as Falco).
he's definitely not given enough credit and an awesome character, but I guess if I had one complaint, it's that his neutral B can only cancel into neutral a in midair 