I was already planning on placing limits on esoteric abilities myself- though admittedly as part of the package deal of being able to purchase "higher" abilities for multiple lesser ones. I.e. if I allow a mortal to get a Divine ability by spending six feats, that's going to count against the number of esoterics the character is allowed to have. My inclination was to limit this to one, but I admittedly haven't investigated it heavily by building characters and seeing what comes out. Honestly, my two powergamer players are better at that sort of thing than I am, so maybe I should hand off the idea to them and see what they can come up with.
As for Artifacts, I would say that any limit on the abilities offered by a particular artifact should be dependent on the Artifact's maker- thus, if the Artifact is made by (or, being blunt, part of) a Deity, then it should be limited to at most one Cosmic ability regardless of how many equivalent Divine abilities the artifact is "worth." This is why artifacts are used, and why they are valuable: they allow the creator to bend or break limits like the one-esoteric-ability rule. Also, I would suggest that
any artifact containing what would be, for the possessor,
an esoteric ability, should carry drawbacks automatically and not always work as intended. The "power with a price" trope is cliché, but it's cliché for a reason. This, in turn, would prevent deities from loading their artifacts up with Cosmics to get power equivalent to a First One (but on that topic I should really take care to point out that a mere 8 Cosmics does
not in fact grant the power of a First One, since the First One template itself grants a Transcendental ability and several other Cosmics the First One doesn't have to spend its own "slots" on), and would actually make them think carefully about loading up with more than one or two Cosmics at all.