Thats the thing...when you do buy the cheap ones, you need to pull them apart, and seal them up properly. It's not a hard thing to do. Also, get some loctite onto the bolts that hold them together.
We've had 2 running for more than 2 seasons in nothing but salt water, and they take an absolute beating. One came apart early on, as it was just fitted straight out of the box. It lasted about 3 rides. The other 2 properly sealed ones have had no problems at all.
The one that failed was replaced no questions asked by the company we got them from. That works for me. In saying that, you can't beat the OEM ones like you said, but I can't justify the cost.