They're too blatantly evil for many to follow their path, which in some respect makes them poorly designed I suppose, as an option. Sawyer has talked about their better aspects on social media but very little of it is in the game and probably wouldn't justify them to many players anyway. Perhaps better to have made them Fallout 2's Enclave and not able to be joined, using those dev resources to enhance the other paths (or create a Brotherhood path).
It's really hard to make an enjoyable "evil" faction to join, in my experience. Somehow Bethesda managed it with the Dark Brotherhood, in a mainstream gamer sense at least. Key ingredient there seems to be lots of cheesy dialog to soften their edge.
To create an enjoyable "evil" faction you first need to make them plausible. No society is evil to it's roots. It's against human morals/ethics and prime insticts to be inherently evil. Caesar's Legion is weird because it is huge army and no one questions crucifixtions, slavery, women objectification inside. I don't remember a single Legion member saying something like "I don't agree with everything Caesar says but he is the founding father and a great leader. I follow him because to do otherwise I would end up like Burned Man, but there will be time where he will have to listen to us" or "Caesar created a philosophy suited for waging wars. We will need to rethink some of our policies, people will revolt if we blindly follow his teachings in the time of peace". Instead you have fanatical zealots even among the highest ranking leaders. Caesar created Legion because he thought Roman Empire is so outlandish and it's customs so outdated that it might work in post-nuclear Dark Ages.
If Legion had more merit, some justification stories about stripping women of their rights, something plausible to why they are bringing machetes to gun-fights then more people would play them. House on the other hand, he is an authoritarian asshole, centered around himself, he doesn't even once from what I remember say that he wants cooperation, instead he wants to be one above all, have the final word when he doesn't even know or care what people on his streets think about him. But he does provide constructive criticism to democracy as a whole, because the other option he sees is NCR. If you reveal to him you support The Legion he will be horrified in his last moments. Also House is believeable option while Caesar we all know will probably scorch New Vegas to the ground, kill everyone who disaggrees with his vision and go somewhere else because his nation needs an enemy. It's best summed up by NPCs outside Legion commenting on Caesar's death and how it will make Legion fall apart in following years - because no one there cares what Legion stands for, they only follow because of Caesar who created this one big snowball.
You don't have to agree with everything I wrote about them, those factions are still interesting and the fact we can hold political/philosophical discussion shows New Vegas is above competition's designs. Speaking of them - let's look at The Institute. In Fo4 their MO is explained by infamous quote "It's too complicated for you to understand". Because Institute is evil and there is no effort made to make them plausible like with The Legion. All it could take, would be the same trans-humanism idea The Master from Fo1 followed. The Institute could be about making everyone a synthetic, equal and stronger than normal humans. There could be a story about them being betrayed by wastelanders on their first contact. Instead we got a lulzy faction which kidnaps people, injects them with a FEV serum that causes painful death or turns them into raging mutants which they proceed dump on the surface to screw even more people and their only justification is "we are doing it for science and you can't understand science".
The "Dark Brotherhood" is the 1:1 copy of Fo1 Brotherhood. They were a technological police back then. As such in Fo4 they had the most merit. They sack the supplies from settlers because they are at war, they wanted to conquer the Commonwealth to confiscate everthing more advanced than microwave so no one would get any funny ideas like The Institute. People compared them to Nazis because Brotherhood feared synths and wanted them destroyed/killed, they saw them as tools of war and something that could one way or the other end humanity - although that is never discussed in Fallout 4 since the developers are on the level that makes them think Kid in the Fridge quest was initially funny, so Maxson was spewing propaganda slogans 24/7. And those who played the game didn't really knew "The OG Brotherhood" and didn't really wanted to dwelve on the AI dilemna.
Anyway House is evil, Legion is cartoonishly evil. It sort of works but like I wrote above to make more people like them and them more enjoyable, there should be some sort of inner opposition like in NCR or New Vegas when we talk about Mr. House.