Li Shenron
Legend
Legacy justifications aside, it is just way too good, and seems a ham-fisted way of representing this idea.
Shrugging off certain effects like poison or mental compulsions is fine, but this is overkill.
Also, I always felt the monk was supposed the guy who was good at 'resisting everything'.
I am not sure it is "too good". There are also other things at high levels that look unfairly good on paper, such as the Monk's suddenly getting 20 in all scores (much later than 13th level admittedly). But we know that ability scores will be very high to everyone at that point, so what looks broken may be not. Other characters may have advantage on several ST from a variety of sources, so it may end up that the Fighter's feature is great, but not broken.