The larger factions with lots of units (BoS, Enclave, Settlers) can kind of do everything and, unless playing Battlemode (which has restricted list building like traditional wargames) also you can mix and match factions as you want unless there’s faction rule against it (Railroad can’t take Institute for example) but even that you can ignore in solo/co-op games against AI.
None are really poor/bad as the game isn’t really that hyper focused on competitive play. If wanting to play the best/easiest to build a good list: at 500 points it’s Minutemen and at 1000 it’s Enclave but even then both are not unstoppable but you have to design your list specifically for them. The type of scenario you play can really change how useful a power “s-tier” list that some people use performs as most only look at how easily they kill things vs run objectives.
Here’s the general playstyles:
Enclave and gunners – squad based shoot everything + heavy weapons. Enclave being better at high point games. Both start out really strong but weaken a lot as units die or if units move too far apart.
BOS: Heavy armor specialists. Can use pretty much anything in game (if running restricted lists) but expensive. In unrestricted (standard rules list building) they’re basically cheaper PA as it’s included in a lot of units point costs at a discount.
Settlers and NCR: Non-heavy armor specialists. Lots of skills and more ranged. Less PA access the BoS but at the same time cheaper units. Settlers focus on unique characters more with lots of special abilities while NCR is more about the generic units being good.
Minutemen: Moving lots of units very quickly and having action advantage. Good ranged. Arguably the best faction at smaller game sizes. Preston is a god in FWW and must play if running MM.
Raider and Legion: Run and stab swam. Raiders really good a low point games like MM. Both have support ranged but it’s not main focus (legion is better at this). Raiders also love chems to boost their effectiveness short term while legion cannot use any drugs.
Super mutants: in-between raider and settlers. Better melee but decent shooting. Really good unique units.
Institute: Honestly couldn’t say as I never use them. Think they’re a mix of low cost range swarm and expensive specialists.
Railroad: Settlers more or less with some of the best unique units in the game. Desmonda and Deacon are really strong. If not playing restricted lists (ie battlemode) it’s generally better to put railroad in with settlers/NCR/Minutemen or your favorite faction (as long as it’s not the institute) as they don’t have cheap generics.
Creatures: Tanks that kill everything. Mostly melee.
Robots: Tanks that kill everything. mostly Ranged.
Everything else is more of a subfaction (like forged for raiders) of another faction or not really PVP viable until more things are released (Children of Atom).
Raiders will likely move into the do anything group over the few months as there’s a bunch of additions coming and likely more subfactions based off of Nuka-world and FO76.