I’ve been in contact with the makers of Nik software. All of the older versions of their plugins work with other than Adobe products, but only 2 of the new versions of them work with other than Adobe products.
With Adobe going to mandatory subscription if you want up to date, rather than a upgrade from one version to another, I (and others) have been looking into alternatives to Adobe Photoshop. One of the front runners is Corel PaintShop Pro X5. All of Nik plugins version 1 work with Corel. Only Define v2, and Color Efex Pro 3 (which has been superseded by v4 now) work with Corel.
Unless they are locked into a Microsoft style contract (your software can only work with OUR programs, and no one elses’) it would behoove Nik to make it compatible with more than just Adobe Photoshop/one photo editing program.