The Brazen Coalition are quite clearly in chaotic good territory for this plane.
Heartless Pillage, most of the Dire Fleet, the various undead/pirate effects (Grim Captain's Call, Boneyard Parley, March of the Drowned), Pitiless Plunderer, and to a lesser extent several other black pirates and/or pirate themed cards would beg to differ. I'd say it's a stretch to call the Brazen Coalition as a whole "Evil", but it would be more of a stretch to call them "Good" as a full group.
Chaotic Neutral, with a stronger leaning towards Evil than Good would be my best call.
I guess that's a good argument, but the Brazen Coalition basically being the targets of genocide, driven out of Ixalan during their non-pirate stage by the Sun Empire's totalitarian nature, and being driven to a set of islands and sinking ships is basically their entire story before turning into those people.
Most pirate undead spells are utilitarian in nature, and don't reflect off the Pirates in question in a way worse than the fate worse than death style that the River Heralds/Sun Empire does, or the immortality/bloodsucker theme that the Legion of Dusk has.
Skullduggery implies that pirates are willing to negotiate with people. In many of the Uncharted Realms stories they are even willing to take on merfolk despite many merfolk being associated with the River Heralds.
Meanwhile the Sun Empire is caught up in TF2 Soldier-level jingoism and nationalism, and the Legion of Dusk is no better.
The only group in Ixalan that is anywhere near better than the pirates is the River Heralds, in which their interaction with other people leaves deadly abominations that run risk of destroying people or wildlife with little warning, has very little of a standing army, and has only shown interaction with the Sun Empire, in which they are extremely far in neutral (if not lawful) good territory with a few (Kumena) hurled straight to chaotic evil.
So not only is the brazen coalition good guys in Ixalan, everyone associated with white mana is deep in the evil alignment except for Azor, which is a questionable figure since we don't know who he is besides someone that made the absolute worst legal organization in the multiverse besides the Phyrexians.
One more thing besides the alignment system in Ixalan in particular, is the fact that there's absolutely no way that chaotic good is possible using any color combination.