"So why do so many have issues with swords?"
If anything, all I see is people praising the inclusion of Pyra and Mythra. It's obvious that everyone LIKES more anime sword fighters (BOOBA!), and the ones who don't like them are just a negligible minority.
So far, I've mostly seen people voicing their discontent regarding "anime swordfighters" being nothing but mocked and ridiculed, as if there was absolutely NO reasons to NOT like characters like Byleth, Pyra/Mythra, Sephiroth (...) being added to the game.
The thing is, there are reasons why its annoying but it doesn't matter because the majority of people want more anime swordfighters. Even if the game would be filled with them, they'll tell you "it's perfectly fine! You're just an 'anime swordfighter' hater lmao".
A someone pointed out, the game would (rightfully) be criticized if most DLC/New character would be FPS characters, such as: Master Chief, Doom Guy, Joanna Dark, CoD Protags, Destiny Protags, and so on.
Now, here's my 2 cents:
When I bought SSB64, back in the day, there was this feeling of getting the apex of what video games had to offer. Each characters were unique, and gave this "grand tournament" vibe. Every fighters were respected equally on their own merit (except maybe for jigglypuff... until rest KO was discovered).
Nowadays, SSBU indubitably feels less so. Series such as Pokémon, Fire Emblem and (arguably) other "Anime Sword Fighters" series take more space than anything else, usually due to their sheer popularity instead of value. The rest of the roster feels more like trophies who are there for the sake of it (Everyone is here!). You can literally browse the roster and be like "oh yeah, I completely forgot this character was here... well, who cares."
Regarding the "anime" part:
When you look at anime (or mangas and video games), you find out it's a highly standardized art style used for good and mostly low quality product/narratives (see: most shonen) because it's a simple art style that can be easily scale up into detailed artworks if needed. And since every artists you hire are expected to know how to draw in that style. And so, the more artist have to know that style, the more it becomes prevalent. Ironically, because of production requirements, the art that can be used by japanese artists became more restrictive, less creative and uninspired. That said, it doesn't the style is wrong by itself. In fact, americans have the same "problem" with the Disnay style, the gritty style and the comic book style.
Implicitly, that's what people try to point out: adding more anime style feels bland and uninspired because, in a way, it is. To quote a friend of mine: "Most people like me really like Big Macs, but they still feel cheap."
As for the "sword fighter" part:
Swords are prevalent in every culture. In a way, a game like smash who aggregate various heroes of various stories and series alike will have to face it in a way or another. Some use this argument to justify or even encourage putting more sword fighters in smash. Then, there are people who disagree and would rather let smash be as diverse as it used to be (or rather feel to be), instead of it turn into Soulcalibur.
Those actively favoring sword characters would argue that sword fighter in smash are already very unique but, if we excludes Links and specials, I do not agree with that at all. Even Soulcalibur, which has plenty of sword fighters in it, has more unique movesets than smash currently has.
Of course, "anime swordfighters" can be unique, but they currently are not as unique as other types of character are between themselves.
As for the "uniqueness" part:
There are many characters (even with swords) who are more unique than most of smash's swordfighters. You could even get characters from already included series. In that regard, there are plenty of options from series like Megaman, Sonic, The legend of Zelda, Donkey Kong, Metroid, (...), that would be more unique merely on the way they move and fight (again, even with a sword or not). Then, there are plenty of series not in smash yet that can bring some fresh air.
Characters like Hero are unique even with a sword (even with the RNG BS). Even if I really don't like him, he's still a good addition to smash because his moveset has a lot of character and is still very unique.
Characters like Pyra/Mythra or Byleth, on the other hand, aren't as much. They either flashy or a melting pot of existing stuff. Once you see beyond that, you realize they don't offer something unique at all, aside for esthetics.
Hero was "flashy" too, but he functionally remained unique after a while you get used to.
Just ask yourself: What do they add to the game? What do they add to smash that stand out enough for players to choose them over the rest of the roster?
I could give you an answer for Hero, Link, Marth, Sephiroth or even Lucina, but I'm not so sure about either Pyra/Mythra or Byleth.
Regarding technical/gameplay stuff:
There are also more technical reasons for why sword fighters being added isn't a good thing. Such as how "safely" they can trade hits with brawlers, thanks to disjoints, or that they can space out and camp easily most of the cast (thanks to: reflects/counter/range/great framedata/projectiles).
In that regard, not only swords are perceived as an issue, but any melee weapon added to smash are potentially an issue as well. Especially if they are swung just as fast, if not faster, as any brawler can punch when the latter doesn't have the same reach as the former.
'Nuff said