Yep. Very true. It can even cause negative resists at 90% weaken which is 90% of the time, quite common but 100% guarentees it and even more dmg. Makes immune enemies not immune and non immune enemies more vulnerable to dmg. 200% weaken is even better bur requires more slots.
Ascendant is a good set affix to go with weaken though for extra reduce resists as it can stack and the hirling debuffs work with it. I did try hirling wrath + weaken to try and buff characters which seemingly works but i doubt it actually works since i remember it was mentioned that it shouldn't work, unless thats changed or im seeing placebo because my builds already too powerful for the floors i go to.