I found that hillside ponds are one of the cheaper options to have dug since the materials can be mostly moved/pushed with a dozer a relatively short distance to form the dam apart from the keyway/core trench.

To mitigate the potential spring or vein issue, you could have the builder dig a core trench around the entire perimeter of the future pond and back-fill and compact good clay materials to make a tight seal all the way around. I suppose you could dig a test trench on the up-hill side in preparation of this feature, and see if you hit any seams along the way and know what to do for sure.

My pond was dug on a hillside, and we did hit some very low-flow seams that just stayed damp. It is possible they are the reason my pond leaks a little, but not intolerably so. I have blamed it on no compaction on my dam apart from a large dozer which isn't enough. My inflow is far higher than outflow, so the lowest I have gotten was 2.5-3 feet down last summer in a record drought.

This year it has not dried out enough to stop running out of the overflow. Rain, rain, and more rain.

As a note, I found that low water in the summer makes for a great opportunity to control cattails. Makes them easy to pull when you can stand on firm ground rather than pulling from a raft.