Everything takes a hit with lower mysticism. It isn't like magery. It's more like magery + eval in a way.
Your resists in stone form take a hit.
Your ability to not be affected by curse (like from Slasher) takes a hit.
Your ability to not be poisoned (like from Miasma) takes a hit.
You have less options for your spell trigger.
Your Rising Colossus is weaker and doesn't last as long..
You have less of a chance for purge/sleep to work.
Cleansing winds heals less damage.
All your damage spells are reduced.
You can get away with 110 mysticism, but if you're putting it on your farmer, can't you make back the 10-14m you'd spend on a scroll pretty quickly anyway and enjoy the full perks of a Legendary Mystic?