I thought the same way until a friend proved me wrong last year. We were both mounted Paladins on a field with a LARGE number of low-level undead enemies. We each cast it on ourselves and then rode the field virtually impervious to AoO. Of course, the skeletons and zombies chased us, and we lead them into the traps and areas of concentrated artillery fire before ignoring them completely to go after the leaders. We also would charge the commanders, smite them, cast Sanctuary as a bonus action at the end of our attack, and then next round, ride away.
Game mechanic: We were Tier III paladins (I think level 12?), and the average skeleton / zombie needed a 20 on the die for the Wisdom save AND a 20 on the die to hit our AC.
Huge undead army and not one reached the castle wall. DM spent hours on his "Battle of Winterfall" scenario, and we "broke it, spanked it, and sent it home without dinner..."
