We've been managing one area of poison hemlock at Quincy Mine dry house ruins for several years. We pull the PH out by it's taproot and bag it. We are now down to several hundred small stems that fit in one large contractor bag. If left, it was heading to be a monoculture-like infestation. I visited an area west of Denver in the foothills this summer and saw crazy infestations of PH, which a friend of mine eliminated from his yard by annual pulling, so yes I have seen them outcompeting natives but on drier, upland sites. And annual manual control works well.
Cheers,
Sigrid