Honestly, it’s probably fine as long as you aren’t applying it to everything or trying to blend many sets of world aligned textures together (landscape materials are infamous for doing this) but at the end of the day what is “too expensive” depends heavily on your target platform, which you will need to profile for.
The idea is to use it only on cliffs and maybe rocks to make them blend better together and the usage would be limited to this specific case. Regarding the Target Platform, we don’t have a specific target hardware yet (But it’s for PC. The hardware Minimum Requirement is something closer to a GTX 1650 and 8 Gb RAM for now).
If you do decide to look elsewhere, I would suggest dithered blending of UVs. This allows you to do triplanar/world aligned texturing in a single texture sample. The sampling costs more but I’ve been told it is meaningfully faster than doing 3 samples.
I’ve tried other solutions like the 1 sample triplanar:
Even though it worked at first, I found it tricky to implement without any issues compared to the more straight forward method available in UE4/UE5 by default. It tends to break when I try to calculate the B channel through shader or do other shenanigans.
Either way, I managed to find some videos explaining the method better and could get similar results with less texture samples. I would like to thank you for the article. It cleared a lot of questions I had prior to this.