Boric acid is used in eye washes, though at higher concentrations at around 1% (1750 ppm). It is a weak acid which in buffered water isn't going to have any "acid" effect on eyes or anything else. Even a strong acid such as Muriatic Acid added to the water does not cause a problem -- it lowers the pH, but it is the resulting final pH that determines whether there is an effect -- the type of acid used or its strength is not relevant.
When added to the water, boric acid is identical to adding Borax and Muriatic Acid (except the latter results in more sodium chloride salt). Gentle Spa IS boric acid with a small amount of Borax to make it a little more pH neutral, but pure boric acid is only slightly acidic in buffered pool water and again that's not relevant anyway. Adding 50 ppm borates using boric acid to water with a TA as low as 50 ppm would lower the pH from 7.5 to 7.15 so not that much considering how much is added. Basically, most of the boric acid remains as boric acid in the water rather than dissociating to become borate ion. At a pH of 7.5, 96.2% is boric acid while 3.8% is borate ion.
Dry acid is commonly used in hot tubs because the quantities of acid needed are small and it's more convenient, but the same concerns for sulfates apply there as well. It's just less likely to find as much stainless steel where the high sulfates might be an issue for accelerating corrosion or for there to be splash-out on cement or stone where magnesium sulfate salt recrystallization pressure could crack such surfaces. Muriatic acid is only damaging if CONCENTRATED acid comes into contact with surfaces. When diluted in water, the type of acid is not relevant to damage (except for side effects such as sulfates vs. chlorides) and it is the final resulting pH that is what is relevant.
So if adding 20 Mule Team Borax and Muriatic Acid to a spa or pool for borates, one should add them slowly over a return flow with the pump running. For a spa, with the small quantities that are used, you have the option of simply mixing them together in a bucket of spa water since once combined the pH should be close to neutral so adding the combo would be safer.