The clay is exelente media, hovewer it has a weaknes.
If it dries including freeze dries it looses its quality.
Meaning it starts seeping and/or looses strengt.
Easy to overcome if you are awere of it, just cover the clay in the areas that might be exposed to drought/frost with other soils to keep clay moist and thawd.
Thats my experience anyhow.
I know that oversatureted clay (quick clay) can/has caused landslides, so if used on steep hills or biger scale dam, that might be an isue, deepending on the shape/profiles/scale on your pond.
