I do not think this match up is 'good' for us per say. I would say it is even. I recently fought False several times and have regular practice against Lucina/Marth. I would also like to add that in the match with Leo vs False at GOML, False was quite close to taking a game off of Leo with his Marth where as his Sheik did not fare nearly as well.
Dancing Blade if timed right can beat dash attack, and of course beats dash grab. His retreating air speed means that it is also difficult to challenge his aerial baits and options, and you cannot challenge directly because his sword is longer. The easiest way to win this match up against Marth/Lucina's who are not as skilled is to wait for them to dash in and abuse their long dash animations where they cannot shield to hit them with a confirm. If you are fighting a Marth that walks instead then it is a lot harder. For these reasons, neutral is a bit of a struggle. Trapping Marth on the ledge is also an issue because of the amount of options he has that beats out range. Ledge drop forward and neutral air, ledge jump counter, ledge jump invincible Dolphin Slash are all mix ups you need to be aware of. They are also one of the few characters that is threatening to Meta Knight off stage as he recovers. A lot of moves beat out Drill Rush, counter included, they have the aerial speed to chase you and get forward air kills off the side, and their forward smash can actually hit Shuttle Loop safely from the ledge without them being at risk. Recovering low and using a reverse Shuttle Loop ledge snap or Dimensional Cape is usually best.
The best way to beat Marth/Lucina is to not give them a neutral once you have taken them out of it. They have issues with landing, so in a juggle situation you should be able to get under them with a shield which covers most options. You also need to know your frame data and how it compares in CQC. For example, if they use a reverse up tilt on you out of shield and you do not know the correct punish, you could get up tilted again, which shifts the advantaged state in their favor. Make sure you get into the habit of always DIing behind them when they grab you, since this will help to prevent them from getting guaranteed follow ups at wider % ranges. If they jab you, jump away and air dodge. Also be aware of shield management. They cannot shield break you with Shield Breaker from a full shield unless you panic or already have taken shield damage, so make sure that you are using it as a 'bar' and only opt for it when its 'full.'
I don't think this match up is worth a secondary, it is just one of the more frustrating ones if you do not play it right.