Somehow for years not many people even in endgame pvp seem to have found this out, but Onslaught states in the tooltip that it grants pen based on the enemy's resists.....however this is not the case as it actually gives you pen based on your own resists.
"This attack ignores the target's Resistance and grants you Physical and Spell Penetration for your direct damage attacks equal to 100% of the amount ignored from the initial target for 5 seconds."
You can easily test this by hitting an unbuffed friend and seeing how your value changes. Then do two following tests where your friend buffs up with resolve and a test where you buff up with resolve. The pen granted will always match your own resists. It has been this way for years, but nobody really notices since most players are relatively around the same resist value of 25k to 30k.
Allowing tools like onslaught or acuity or corrosive to bypass the stat system can quickly be a slippery slope where combat boils down to everyone ignoring traditional combat in favor of waiting for cooldowns every minute. It would be far better to make players work for a bonus like perfect pen and give counterplay to it.
For example I would bring the uniqueness back to onslaught where you gain 5s of 5000 pen and grants the ult cost back if you secure a kill. The bonus continues to stack pen if you can close out another kill with the ult. This way the caster has to combo correctly in the first place to even get the buff and then continue to work for the pen instead of just waiting for the ult gen cooldown. On the defending party side, they have many opportunities to dodge, block, stun, or heal to shutdown the caster.
We should use the insightful and awesome buttons more