Piercing is a tricky damage type because of the mechanics behind it. Armor piercing converts physical damage into pierce damage at the given ratio for a weapon. However, it’s very difficult to increase that ratio with one major exception, the blades of nadaan constellation. It will double the armor piercing of your weapons if you have a sword equipped.
As for viability at late game, it is viable but it is also harder to come by RR for piercing than many other damage types. You can use acid splash from the manticore constellation and night’s chill from the nightblade tree which comes out to 50-60% reduction, which is not bad but not exceptional compared to the much higher values you can get for other types.
The real trick is in getting the piercing damage itself to start with given that many weapons have a 25% or lower piercing ratio. Even doubled, you’re still losing half your damage to basic physical.
Acid/pierce is an option, chaos/pierce I suppose you could do but it would be better as just all chaos. Cold/Pierce definitely has the best item support in the current version of the game but who knows what the expansion will bring. The possibilities are limitless.
Two things I didn’t mention. Dermapteran Slicer has a 50% piercing ratio which can be doubled to get full conversion, and the Valdun’s set has a piercing boost to the already very high base pierce value of 50% but is still short of full conversion since you can’t use blades of nadaan with a rifle.