But... what are they? very few devil fruits in general even have weaknesses and when they do even then they tend to be rather hard to exploit. The easiest to exploit so far is crocodile's since all it takes is water in a world that is mostly ocean...
The first weakness would be the need to create a "room" before having access to the powerset. It doesn't take more than a couple of seconds but that's more preparation time than any other DF. That also means it's the weakest DF against fast surprise attacks unless the "room" was prepared beforehand for some reason.
I'm using the premise of "Me eating the DF", me as a person with no haki and no superhuman abilities and certainly not the training Law underwent from the Donquixote Family.
The other big weakness of the Ope Ope is the amount of multitasking required to control abilities like "shambles" the way Law does, and moves like switching bullets with snowflakes require superhuman speed, reaction time and thinking. (The switching power in specific is limited to switching only objects of similar size, and that limits a lot the ways to use it defensively because he must be aware of what attacks are coming, their size, and where are objects of similar size that could be exchanged with either his own body of the projectiles). It's a tricky DF that involves a lot of strategy to be used in a big fight.
Of course if we're talking of just taking care of a couple of fodder thugs then the fruit is always OP, but that's different form using it against Smoker, for example. I bet Smoker would kick my ass if I tried to fight him with the Ope Ope.
To me (just my opinion here), other Paramecias like Mr.1's or Foxy's or generally any other that comes to my mind is more straightforward ("I turn any bodypart into a blade and cut you with whatever hit I can land of you", "I can shoot a slowing ray from my hand", "I create weapons or traps of wax", etc). Of course strategy can be applied to any fight no matter the fruit, but the use of most Paramecia's power seems "easier" than Ope Ope.
The way I see it, Ope Ope is more of a fruit that provides you with "tools" (the tools from an operation room) but requieres the user to be intelligent, fast and efficient with them.