But CLU is OK, just a PITA to fully remove from the surfaces if you want to do a remount.
Indeed. Obviously you don't want to use it with aluminum, but anyone serious about cooling is using something with a copper base/block anyway. That being said, it does "corrupt" copper, making it necessary to do some serious lapping to get rid of the stains, IF that's what you want to do. In my experience, there is no performance penalty in leaving your copper stained. I am not sure if it does the same to the nickel coating on CPU IHSes, but maybe once I replace this Sargas, I'll lap it beyond the nickel and see if the CLU got into the copper.
And, as I have stated (and to restate what you said), Liquid Metal Ultra (aka CLU) does not "weld" or permabond anything to anything. Liquid Metal Pro can sort of do that, making it difficult to physically break the interface. Some say that Pro is a bit better, but it can be difficult to apply since it beads up. I believe the recommended method of application is to spread it with the tip of a syringe needle. I used el-cheapo watercolor brushes from the Target $1.00 aisle to apply CLU in truly miniscule amounts, and it's great. Five years later, the application is still going strong.
Also, I noticed that, unless you have some serious contact issues between your IHS and your base/block due to warping, that lapping may be less necessary than normal when using CLU. It has such a high thermal conductivity that evening out the surface texture won't help as much as with a paste that is highly sensitive to bondline thickness.