It's not just carbon fiber posts. I got a cheap Thomson knockoff for $10. Looked fine and I'm fairly certain the post itself was strong enough. The mechanism to hold the seat was a very underbuilt 2 bolt style. It failed on my first/only ride on the post. I sort of wasn't stranded but took a while to get back. It is a design issue, it might work if you have the capability to remake the mounting hardware from steel.
If you browse eBay, a lot of the metal posts, Al or Ti are this style. I wouldn't expect any of them to survive a hit/pothole/washboard. The carbon ones have a different set-up.
I'd go for a cheap carbon over cheap Al for this reason.