Both of these S-Grades are sought after, but you have no choice other than dealing with the RNG. Many people have suffered as a result thanks to horrid RNG, and it hurts people that don't have as much time to play the game every day.
S6: Lucent Domain - Currently only drops from Armada Training: Final Lament and Crimson Spark Trigger on Global, both of which have extremely low drop rates on this S6. Even on JP, which has another trigger that Global never got, the drop rates are awful...
In my opinion, S6: Lucent Domain needs better drop rates and more sources. A tradable capsule dropping in content would be nice too.
S6: Skillful Adept - Highly sought after, but no capsule means that players who are unlucky can find themselves struggling to get it. Even though it has quite a few drop sources, it's not particularly common.
In my opinion, S6: Skillful Adept deserves a tradable capsule dropping in some content at this point. In addition, I feel like it should be able to drop on rear units in Divide Quest. It's far too popular to not have a capsule like Skydance Support and Still Grace, though.
I'm personally making this post frustrated about Lucent Domain, but I wanted to point out Skillful Adept as well, as there are many people I know that have grinded hard for it with no results to show. I've personally run several hundred Crimson Sparks running every single thing I can to boost my rare drop rate, including several EX Triboost 200s. It's not only getting frustrating because I'm not getting anything, but finding people that actually want to do it is getting harder and harder every da. And of course, there's no RWB shop for Lament, and even if there was, getting people to run it that can S Rank it... oof.
There are other S-Grades that are pretty damn rare without a capsule available, but they have replacements that provide similar effects or don't impact damage as much as others do at least.