They're brittle.
It's very easy to end up with voids and faults included in the casting that will give it an easy shear line to crack upon.
There ARE high quality castings which do not suffer from these problems, but the "cheap" castings out there are now aren't it. I've never heard of problems with the pre-ban Essential Arms cast lowers, other than being kind of ugly - but they're plenty strong. I've heard they were investment cast for EA by Ruger.
I know of several instances of both Olympic Arms and Hesse Arms (now Vulcan Arms) cast lowers cracking right in two while being handled or fired. At least with Olympic, if it happens to you, they'll replace it free of charge, and you'll probably get one without any faults, if not the first time, probably the 2nd time around. With Hesse, you'd be SOL. I've never heard of problems with the DPMS cast receivers - but I don't know how they price out compared to the Olympic and Hesse.