I switched from UH21 to US11 on my VJ-626UF a couple of years ago.
The UH21 ink was great in many ways, the colors were very good and the details I could produce were amazing.
I very soon found though that the ink was extremely rigid and brittle, and it also continued to harden when exposed to sunlight which made stickers crack and curl if applied outdoors.
The flexible US11 ink is better and obviously more flexible, but unfortunately it exhibits the same problem when exposed to sunlight for a longer period, it becomes harder over time and breaks.
US11 is not as scratch proof and not as solvent resistant as the UH21 ink, but it is softer and has a rather nice semi matte and smooth surface that I like.
The color space is also a little more limited than the rigid ink, but I can live with it.
I can not say whether the US21 ink will be suitable for your needs or not, and I suggest you talk to a Mutoh reseller.
Maybe they can provide you some samples?