Part of the trouble is that no one's ever really captured a good distillation of what "Incarnate Law" is. Modrons and Formians are representatives of Law as Hierarchy, but that's unsatisfying. I mean, even the Slaads have a hierarchy so it's not especially unique. Another issue is the very good question of whose law are they the incarnation of. The laws of angels and devils are massively incompatible. Inevitables try to sidestep this by invoking some sort of abstract Cosmic Law, but too often that leaves them only interested in boring abstract problems.
The perennial problem is that in the modern D&D cosmos Good and Evil are fairly well defined sides with clear motivations and traits while Law and Chaos are not. They haven't had a well defined role since way back in the Basic D&D and Greyhawk days when things were a lot more Moocockian and Mordenkainen's hardline Neutrality was a principled stand between equally hostile extremes.
So really, if you want Law and Chaos to be meaningful players in your campaign, first you need to define what they mean in modern terms. Are they Stasis and Genesis? Stability and Destruction? Hierarchy and Anarchy? Some mix of all of the above? Define your terms and I suspect a good candidate for promotion to the role of Champions of Law will present itself.
Even before Law and Chaos in D&D were associated with Moorcock they were drawn from Anderson. Read Three Hearts and Three Lions. Law was in general aligned with good and Chaos with evil. Exceptions were quite notable. Ra Partha had a miniature of a "demon of evil law" for example, and there were creatures noted as being "chaotic but good" or "lawful but evil". This was back in the days when there were only three alignments, Lawful, Neutral, and Chaotic.
I have read it, though it's been like 25 years since then. My extremely fuzzy recollection is that Anderson's Law and Chaos were indeed a lot more explicitly Good and Evil. THaTL is where the inspiration for Paladins comes from, after all. Moorcock's Law and Chaos are much more relevant to a discussion about the two as cosmic alignments orthogonal to Good and Evil.
Yeah, honestly other than the LABEL of "Chaotic" the Slaadi alwaya struck me as basically "generic dangerous-hierarchical-Outsider-race" and the "hierarchical" part being distinctly non-Chaotic. Besides the statblock... remind me: where's the Chaos again?
I don't find genuinely "Chaotic" creatures to be a credible threat, because they cannot ORGANIZE (or they aren't genuinely Choatic).
Except of course, in Numberless Hordes of rage/hunger/etc; that's a credible threat! And BossMonster individuals, possibly with wild, unpredictable, and even "impossible" BossMonster Chaos Powers.
Meanwhile, I dislike chaotic neutral outsiders from the other direction: too much obsession with chaos as randomness rather than chaos as an alignment, specifically representing individuality, creativity, and freedom. So while a chaotic neutral human might be a gambler, an artist, or a thief, chaotic neutral outsiders and locations are all 'lol random'
And frankly the true neutral "keep to yourselves mortals vs obsessed with a weird cosmic balance outsiders" isn't much better. In comparison, CE mortals and outsiders both tends towards being viscous marauders, and a paladin and an archon can agree on a lot.
If I could somehow remake D&D from the ground up, I think I might make fey the overall neutral outsider category, with seelie fey as LN, unseelie as CN, and...well, I am not sure what I would call the true neutral ones, but I think I have ideas about overall characterization.