Shield is the biggest problem in mid level campaigns where low level spell slots are not at a premium when playing with who lack imagination in terms of combat tactics.
If you are an 8th level party and the DM insists on whacking on your 27AC with attacks it is a problem. At high level it is less of a problem because monsters will blow right through that 27AC. A low levels it is not a problem because there are not enough slots to make it a problem.'
Also you don't need to disengage, if he used shield, he does not have a reaction and you (and every other enemy) are free to just walk around him and beat up on the squishy character in the back.
An even better tactic for the shield problem characters is to walk away without disengaging BEFORE they cast shield, preferably right after their turn. Then if they take an AOO on you turn around and wail them while they do not have a reaction. This really puts their PC in a bind because most high armor + shield spell characters are designed to tank and tie up enemies, by leaving them without taking an AOO you are really bypassing what their build is supposed to do, byb taking an AOO you and everyone later in the initiative order can attack shield spell free.