First up, a house rule adhered to fairly strictly is the keep ships in squadrons and sub-factions. This means for Imperial and Rebel squads, either no squadron mixing, especially for the same ship type, or balanced mixing as relevant, but for Scum it includes added sub-factional limits.
The rules are;
Empire and Rebel squads may have a single Scum faction as an ally or mercenary, at up to half their squad points. This must be declared before squad building or random selection takes place.
Scum may mix any number of mercenaries in with a single faction, but may not mix factions together as they are natural enemies. This must be declared before squad building or random selection takes place.
Variety is the spice (runner) of life.
The Factions;
Cartel (Hutt, Broken Horn or similar): C-Roc, M12’s, M3’s, HWK 290 (Spice Runner), Y Wing (Thug).
Black Sun: Khiraxz, Z95, Star Viper.
Binayre Pirates: Firespray (Scarlett), Z95, C-Roc.
Lok Revenant: Scurrg Bombers.
Concord Dawn: Fang Fighters.
Tansarii: M3’s.
Mercs, Villains, Hired Guns: All the rest including the C-Roc.
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The Cartels are known for using Mercs regularly, but also have some in-house muscle. With ships ranging from the C-Roc down to little M3’s they can field anything from a swarm to a 2 ship power team.
Black Sun are represented by an even mix of swarm and Ace fighters. The slippery Star Viper or solid Khiraxz squad is a possible core, but a few Z95’s with dirty tricks loaded (EMP, Deadman’s Switch etc) always offer cheap fire power and control. Their best squad is likely a little of everything supporting a strong ace.
Binayre Pirates are almost a set list. 4 cheap Z95’s (all they actually had) and Kath Scarlett’s Firespray, The Marauder are a solid if predictable squad, with the C-Roc and hired mercs/thugs as their options.
The Lok Revenants (Karthakk Pirates) are as much freedom fighters as villains. Their monopoly on Scurrg Bombers makes them a strong faction to ally with or they can be viable on their own. Strong on a ship by ship basis, they can definitely benefit from a little variety.
Concord Dawn are similar to Lok in both squad dynamic and philosophy, they are a brutal single faction option, but leave room for a merc or two (especially a Mandalorian) to add variety to an already lethal team. As one-off or paired hit men, they also strengthen slow or predictable squads.
Tansarii. The M3 became famous at the hands of the defenders of Tansarii Station and most of the available pilots trace their roots back there. They have a Rebel/Tie swarm team-work vibe. This does not mean they cannot have options though. Backed up by a strong merc or a team of Khiraxz only makes their swarm more viable.
Freelancers. The sheer variety of mercs and villains is what makes the Scum so unpredictable. The Mercs can add muscle, unique tricks and traps, and the dynamic a faction is lacking, so become aware of what each ship and Pilot offers.
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The Scum faction has variety and faction exclusive upgrades as their edge in BB (and X Wing). Salvaged Mechs, Illicit and more Crew than any other faction, added to the many-faced dynamic of their ships, gives the Scum player more options (headaches) than either of the other factions. The trick is to create strong squads that keep that unpredictable edge, but avoid the cluster of ineffectiveness than can result.
Half a dozen one-off effects or too much complication will force you to rely too much on luck or superior flying. Trying to build conservatively will hand the stronger core synergies of the Empire and Rebel factions a free game.
Build to a theme (trap). Strong main line fighters, huge swarms, or muscle ships to start with, then add in some of what makes them renown in dirty tricks.