Him being intentionally vague about "I've seen some shit." worked fine for years.
His problem was being slow to adapt political tactics. When they started making real progress and changes he needed to have a serious sit down with the council and spell out the REAL trouble with magic. Death counts. Names of cities burned. What policies failed and how to avoid it.
One of the best lines Jayce has is "Humans don't live for centuries." Heimerdinger can't keep up with the speed of humans.