[Firehed]

Gearman is pretty solid once you have it working, but it can be a huge pain to get to that point. We spent probably two years (on-and-off, of course) tweaking code to get our framework talking nicely to gearman. PHP's gearman-manager library is a little... wonky, and we were seeing no shortage of bizarre APC interference.

Although in this case, I imagine the real problem is portability. Curl is available pretty much everywhere. exec (or more directly, pcntl/posix extensions) aren't in any out-of-the-box installation, and anything that needs to be further daemonized to get up and running (such as gearmand and gearman-manager) are even harder to use in a one-click solution.