Shieldbreaker made my game much easier! Like, a concerning amount of easier. It's balanced mostly by her nightmares if you take her camping(you'll have to even on Radiant/easy) which are a hassle, and almost always spike her stress up hard. It makes her leave any outing worse than she's gone into it and forces a break from her pretty easily. For more hardcore fans they'll probably see her mechanics as just as good as other existing characters they've already mastered, newer fans will probably be a little irritated by how good she is but being locked behind a paywall.
Tips and tricks for the interested:
-I loved using a modified Coach to get a Lvl 2 or higher Shieldbreaker just to send on short missions to level up the rest of my fodder-er, my up and coming group. It avoids camping and makes the best use of her in my opinion
-Like the Jester, she's best dancing from position to position. Keep her with a character the also has moves the move them forward and backward for maximum potential
-Aegis Scales are really good items, farmed or just hoarded for the right moment