Reading through the Classes and their corresponding Archetypes, the Relic Channeler really stuck out to me.
From a logistical standpoint, from an adventuring standpoint, and from even an RP standpoint, it seems like it just works much better than the base Medium.
There's no need to find appropriate locations to channel Spirits; since you always have the same 1-6 Spirits on you, you can more-easily focus your character on only using 2-3 Spirits, or even just one Spirit almost singularly.
You don't need to remember which Taboo you decided to use each day, instead just associating each Spirit with a specific Taboo; at the same time, you don't need to reassign Spells or Skills, either (this might seems more limiting, but honestly it saves a TON of time and bookkeeping, since you can just have a quick reference go-to for each Spirit).
You can actually make the Spirits into long-running pseudo-NPCs, as well, since you'll always have the same Spirits at hand.
You gain Alertness whenever you're suffering an Influence penalty from a Spirit; you gain more Spells, more Proficiencies, more Feats, and more Skills from your Spirits.
And with Location Channel being replaced with Object Reading, you don't miss out on much as far as being able to gather information.
Yes, you miss out on Haunt Channeler, but that may prove next-to-useless if your DM doesn't include Haunts in their game.
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In summation, what you trade out is fairly minimal, and what you gain in return is a much-more usable class.
It may seem like a major limit to only have the same 2 Arcane and Divine spells each level every time your channel, or the same 3 Skills, but honestly you'd probably find yourself gravitating towards those Spells and Skills anyway, excepting for very, very rare circumstances.
And since you don't have to go running around all of kingdom-come trying to find a proper Location to Channel a Spirit, nor have either you or the DM come up with a Spirit's soul & personality on-the-fly for that Location you're in at that exact moment, an RP-based character will have a much greater chance to develop a whole bunch of characters, motivations, etc.
I'm not sure about others, but while the basic Medium is pretty decent, the Relic Channeler just seems to blow the baseline Medium right out of the water in practice.
I think that, for a non-occult game, the relic channeler is going to work much better. But for an Occult game, where you might stay in just one city (or a few cities) and will be dealing with haunts and varied challenges that benefit from the base Medium's versatility, it might not be the best fit.
Some players wanted a class that reinvented itself every day, and others seemed to want a class that had a few distinct "modes". With this archetype, both camps can be happy. I for one love everything about the Medium, and can't wait for a chance to play it or any of its archetypes.
It seems to me like it would depend more on how stable each day of adventuring is expected to be. For something like Curse of the Crimson Throne the default Medium would be great. For something like Reign of Winter RC would probably work out better. If you have a nice DM, too, I think the regular Medium is pretty strong on a daily basis, though if you do have a DM that likes to put the kibosh on things or suddenly limit your options when it's convenient, RC gives you that stability.
I don't think it's as clear-cut as Barbarian v. Invulnerable Rager.
For something like Reign of Winter RC would probably work out better.
I would actually love playing a non-relic channeler medium in Reign of Winter... It'd be great suddenly get characters from earth legends once you get to Rasputin Must Die, and the character suddenly be confused since they're invoking alien spirits like King Arthur and Heracles.