Not alot of big cons to them. I will try to write some down. Of course just my opinions
I have clams.
Possible down sides.
They can get pretty massive, If one dies it can quickly degrade water quality.
Some fish that dont bother corals will bother them.
They build a heck of a skeleton, several large clams could take up lots of Alk, and Ca.
You could get addicted to them, and spend all your money on clams.
They take up quite a bit of space.
Thats about all I can think of.
I like them, and think if you have the lights for them, they are a great addition to the reef. Really enjoy my three.