UngeheuerLich
Legend
Your AC can't be less than 16. THAT IS ALL IT DOES.
It's an incredibly simple spell that needs no interpretation, but people persist in trying to "But Whatabout ...?" it.
No matter what armor you wear. That is an additional descriptor that makes it more complicated and leaves it open to interpretation. So if it had less words it would be easier but still counterintuitiv.
Allowing it to stack with conditional bonuses would make it easier to explain.
Or actually rewrite that spell. Make it: you treat it as chain armor (AC 16 no dex bonus).
Even if it now has offensive uses against very dextrous heroes, it has concentration requirement so what.