The Reaping Mauler's a trap because its abilities pretty much useless. They're incredibly niche to the point that you'll hardly ever be able to use them. Also, they depend on grappling rules, which suck. And they're slow.
First, all its abilities revolve around grappling. There are too many ways that can be rendered useless. Freedom of Movement alone makes every trick the class has useless. Overly large opponents can't be grappled. A lot of the enemies you can grapple have extreme strength and high fortitude saves, so your chances of successfully grappling 'em are fairly low and a DC... 17, maybe, fortitude save isn't much at all.
The big one, however, is this. It takes a long time to pull off the Reaping Mauler's only trick. You need to pin the enemy. That means taking multiple rounds in which you must win every grapple check, just to force the enemy to make a mediocre save, which they'll probably succeed on, and as your level goes up, the ability becomes less and less effective, since enemies' saves increase. And you can only even face off against one enemy at a time, spending 5+ rounds sitting around with that one enemy, waiting for it to fail its save, in a game where the party is usually very much outnumbered.
In general, Monks hoard pretty much all pugilism while sucking a lot and make it an annoying mess to make a good pugilist. Sacred Fist is a good class. It keeps unarmed damage progression and gets a lot of spellcasting. A Monk1/Cleric6/Sacred Fist10/[Anything that gets you 3 more caster levels]3 still gets level 9 spells, you have your casting stat pouring into AC, you have some stout buffmongering going, et cetera, all without leaving the religious concept of a Monk.
Alternately, if you can get the prerequisites for Ur-Priest on a Monk, then use Ur-Priest casting to qualify for Sacred Fist, two levels of Ur-Priest and ten levels of Sacred Fist will get you ninth level spells by level 20 and still leave eight levels to do with as you please, even if those classes don't have any casting to speak of at all. The skills are a bit of a bitch, though. Between Swashbuckler and Monk, you can cover most of the ridiculous skill requirements except for Spellcraft and Knowledge: The Planes... which is actually a bigger kink than I would hope... Oy, what could get those two without being too random?