Hi all! We most definitely do test all the assets, but of course the testing isn't an overnight or instant process. Corona and V-Ray handle and import IOR a little differently, and we've been working on ensuring those imports work equally well in Corona and give comparable results as when imported to V-Ray. It's an ongoing process though, both the testing of assets and the adjusting of the import.
As part of this, we'll be hiding some Cosmos assets from the Corona Cosmos browser next week, ones that are still not importing to Corona as well as we'd like. That way we keep the visible assets to the ones that are working well, to save annoyance of importing something and finding it needs manual adjustment (the same process has been done before, there was an earlier thread from a month or so ago about some items visible in V-Ray Cosmos that were not visible in Corona Cosmos, which was part of this same ongoing process).
Which is all to say - we are aware, we are working on it, and don't panic if some things "disappear" or show up for one engine but not the other for a while. And of course we do welcome reports of specific models or materials that don't seem to be working correctly for you, that can also help guide us to further fixes and improvements. Thanks!