Being one of the first SP free Fighters that Players can get their hands on, SP SSJ Goku RED, he's unsurprisingly been power crept to death and needs a balance change at this point. It's hard to even justify him as a bench.
Even when SP SSJ Goku RED is not viable in the core, he's still an OK bench if the Player is missing multiple Fighters. Saiyan is still one of the better Tags in the game.
No mans land
SP SSJ Goku RED is simply too old to compete anymore and has to wait for a balance change.
Again, he does Buff the Tag, but is hard to justify even here as Saiyan Z-Abilities are a lot more prevalent than they were in the past.
Despite his double Strike Arts Cards, SP SSJ Goku RED is a Blast Fighter. He is primarily used as a bench for his Z-Ability these days, he but can be brought into a Match if the Enemy Fighter has a lot of Yellow Fighters or lacks Blue Fighters.
This Equip is excellent for him, as it gives him the Blast and survivability he desperately needs.
+35% to Blast damage inflicted when allied "Character: Krillin" is defeated. Applies the following effects to self per defeated battle member when this character enters the battlefield: +20% to damage inflicted for 15 timer counts. Restores own health by 15%.
Applies the following effects to self once 5 timer counts have elapsed after entering the battlefield: -10 to Strike & Blast Arts costs for 15 timer counts. Reduces damage received by 30% for 15 timer counts. +80% to damage inflicted against "Tag: Lineage of Evil" when 2 allies are defeated (cannot be cancelled).