Chrom is Roy with Marth's range, Marth's tipper killpower, and a much, much, much, much better version of Ike's up special that deals a whopping 26% damage (TWENTY-SIX PERCENT) and can kill opponents at 0% even when they're onstage sometimes.
Lucina is also dumb, but not Chrom level dumb.
Yeah but... no? Marth's tip Fsmash will kill from the center of FD at 70. Chrom's won't until 90. His power is like slightly above average. Up B is a nice OOS option, but mediocre at best for recovery. And unlike Ike, he doesn't have a horizontal recovery move, so his recovery options are very limited. For every time that you get a weird gimp kill with it, there's probably at least ten times that you die because of your linear recovery. Neither Chrom or Lucina has been placing especially well at tournaments, and most top players consider them high tier at best. Can't see anything that destroys the balance of the game from them.
They're not quite overpowered, but they are annoying in that they take away all the skill required to play Marth/Roy (not that they require too much to begin with) and have almost no trade-off. They shouldn't be as powerful as they are if they can just hit you from wherever with no need for actually decent spacing. Also cut Chrom's sword down a little, that shit has way too much range.
Chrom isn't as easy to gimp as you say... His recovery is linear, sure, but it has gigantic hitboxes and if you're caught by a single one of them you're dead at 0%. His air dodge also goes ridiculously far. Sure, he is gimpable, but the move is still insanely good, in a way that makes absolutely no sense - how come another character in the game has the exact same move, except slower, weaker, with far less range, and all that in a character who is already much slower than Chrom? Sheesh, it can't be that hard to balance it.
And um, yeah, the Marth tipper thing was an exaggeration, but still. If Chrom hits you with a forward tilt at the ledge it kills at 80%. If he hits you with a jab (A JAB) at the ledge you're dead at 70%. A JAB.