I ran an experiment where we blocked the skin sensations with topical anesthesia and then recorded the responses of individual neurons to tES. Nothing changed, so the sensation is not necessary for its effects.

https://journals.plos.org/plosbiology/article?id=10.1371/jou...

We also created the same sensations elsewhere on the scalp, and this was not sufficient to cause neural entrainment either.

https://www.pnas.org/doi/10.1073/pnas.1815958116

Placebo/skin effects can certainly mess up a readout based on performance or behavior but they aren’t the full story either.