Does power surge generate entirely random from the artifact pool?

I feel like getting "weak" artifacts from early and mid-game power surges (silver key, cats claw, sling and scorched spear).
The more powerful have been lategame (after casino).
On the other hand my sample size is very small and they are still very rare.

I have a vague memory of getting preserver in ud once but thats about it.

Sure silver key in ud can be a real life saver for the early game but I've had it as my two recent ones and it does loose its value rather soon.
One scorched drop was for a duellist...

Maybe its that quite a few of the artifacts are a bit crappy and the chances of getting a decent one is slim.