> I helped build earthen structures with sharper slopes than 25 degrees 30 some years ago that are still holding back ponds today.
I think you may be mistaken - note that a "25 degree slope" is just a different way to say a 2 to 1 slope (2 feet across, 1 foot up). I've seen 2:1 slopes used for highway embankments, but earth fill usually specified as a 3:1 slope (18 degrees) - eg when I worked with an earth fill dam (holding back water, same as yours). I've only seen anything steeper (1:1, 45 deg) used a as a temporary (during construction) condition.
Sheepsfoot rollers are good for packing very fine material (silt, clay) but not very good for packing larger granular material (i.e. crushed stone/gravel). Silt and clay are very water sensitive materials: each has a very specific moisture content where it can be packed properly, if your material is outside of this narrow range it will not get to maximum compaction (and therefore it will eventually settle). Water moves very slowly through clay so if it is too wet or too dry it's very difficult to get it back into the proper moisture range, and if there's a bit too much sun or some rain between excavation and placement it will not get packed well. The only reason I've seen clay-ey earth intentionally used is for inhibiting water movement, IE an earth fill dam - and even there, it was a secondary barrier if anything happened to leak through the barrier membrane.
Crushed stone is (relatively) very easy to get to maximum compaction, and if it sits out for a while and gets too dry you can just hit it with a water truck before placing. It packs quickly and easily, and is stable at the ssame side slopes as earth fill