Grapple only really starts to break down once you start encountering CR 5 or greater creatures built around it, and its basically a case of positive feedback.
Your grapple modifier is based on your Bab, Str, and Size. For most PC characters, this is not especially overwhelming. However, there is a problem when you consider most Giant type creatures. Giant types are going to generally have high HD, and at least a decent BaB relative to their CR. Giant types always have a very high strength score. So a Str 30 size Huge creature is already playing with a +22 modifier to an opposed roll. It will almost never fail.
And when your grappled, your actions are basically limited. No movement, and any attack or spell type action is going to require an opposed grapple check, so again, it will never fail.
I have decided that for my own games, the size modifiers for grapple will instead apply to the touch AC attack to start a grapple rather than to the opposed roll. This makes it quite difficult to make the initial touch attack, but gives most fighter types at least an even chance at breaking the grapple, and Rogues / Clerics at least a plausible chance.
Given how spell casters are built though, Grapple is still the best 'screw the caster' option.
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