I'm with Nova ^^^
Extremely invasive ... I have two 5-8 acre areas that it took over. It spread like crazy. The areas became monotypes as the ragweed grew 10'-12' tall, stem density was a stem every 2"-3" and the canopy effect allowed no light what so ever to reach the ground. The only animal sign I ever saw was an occasional deer trail winding through, no bedding at all.
The only way it could be cut was with a large brush hog mower, stalks were tougher than corn stalks and would puncture atv tires. The stalks also would get jammed in under carriage. Gly would knock it back if you could hit it early, but the seed bed was so large that it would rebound very quickly. Seed dispersal was crazy as it popped up everywhere.
I don't have allergies but the pollen would drive my eyes nuts with irritation.
I would rather add bush honeysuckle than ever consider giant ragweed ... other than the above, I am okay with it