If "becoming too powerful" was a genuine concern for the designers, we wouldn't have an entire skilltree of "Mindflayer power-ups" and a whole collection of house rules and/or incorrect implementations making our characters far more powerful that they are supposed to be at level 12.
+1
Thief gives another bonus action, haste gives another action that can be used for anything, . . . the list is endless.
Prof. Dr. Dr. Mad S. Tist 
