Not sure why a +2 bonus would break D&D's balance, but I'll be glad to hear it.
Read about bounded accuracy online and the difference between 5e and 3.5.
Adult Black Dragon has 19 AC in 5e and 27 in 3.5 (Great wyrm in 3.5 has 42 AC and highest AC in 5e is Tarrasque with 25 AC).
Single +2 bonus doesn't break anything, but being able to stack more of them does. Stacking is a thing in 3.5e, bounded accuracy is a thing in 5e.
