Ok. This is getting ridiculous.
I am seeing the same arguments being posted over and over and almost all of them are irrelevant or misinformed.
I don't like the idea of a criminal faction.
Irrelevant. If you don't like it, don't buy it. If you REALLY don't like it, decline playing against anyone who plays the criminal faction. Don't let your opinion ruin other people's fun.
A criminal faction does not have power on the scale of the empire or the rebellion and therefore should not be able to fight against them.
Criminal organizations in Star Wars control entire planets, have their own private militaries and fleets and have been shown to own and use hardware on par with anything else in Legion scale. This is irrelevant, however, because Legion battles are not even platoon sized. If you think that a criminal group that can dominate an entire planet can't put together 30-ish dudes with a crew-served weapon or two and maybe a vehicle then you need to watch return of the jedi, clone wars or rebels or even just look at what cartels in south america do and that is with a single-species, single planet scenario!
A scum faction would just be a bunch of mismatched jawas, ewoks and mandalorions.
There are many different units that could be used for legion that would offer interesting mechanical choices and not be a complete mish mash of civilian or tribal units. Weequay pirates with long rifles, mixed group of criminal thugs with knives and pistols, gammorean guards that could be an analog for wookie warriors or imperial guards, IG-RM thug droids, hover tanks, swoop bikes, gun emplacements, etc.
There should be a bunch of scum factions, each themed to a specific group.
The other two factions don't do this. We have wookie warriors that can fight alongside rogue one pathfinders, Jyn Erso can fight alongside Jedi Luke who had never even swung a lightsaber till after she died. There is nothing stopping people who want a strictly themed force from making one.
Scum units should have restrictions from working with thematically different units.
Why would you cripple one faction with a restriction that other factions don't have? Let's say they had three subfactions. They would have to release three times as many units for scum as they have for rebels or imperials to make the faction competitive. Instead they could just make a commander who synergizes well with his thematically matching unit like Jyn and pathfinders or Han and Chewie or Palpatine and Royal guards. If you can't get around the idea that a group of criminals might hire some mandalorian mercs for a job then you don't have to use them.
Not everyone wants the same thing. Don't exclude people just because they want something different.