As we are but a small island that is the UK, I don't have any real complaints about the climate as we can grow most things. The zone system does not work at all. How can some of the Scottish islands be classified as the same as some of your American cities? The Scottish islands have no trees and are battered by the Atlantic in winter.
Main problems are Digitalis or foxgloves as they grow everywhere and are a pest! Fraxinius or ash trees, they too are a problem. Horseradish as it grows and grows and spreads. As much as I like horseradish with my roast beef, I don't appreciate it everywhere. Salix species or willows another nuisance. Dog roses, oh my, these and blackberries get in all places.
Bindweed or Convolvulus, that tends to get a grip anywhere it can.
Grey American squirrels! Luckily these are under law a serious pest so can be dealt with. Deer the second most destructive thing after man. They too can be dealt with.
The most annoying thing is due to our inclement weather, are dog owners. Instead of taking their dogs to the parks, which are all over here, they simply walk them around the block without a lead. Then let them foul anywhere. Although against the law, they take no notice at all. So to do any gardening at all. you have to clean that mess up first! Plus on the pavements, for children to walk in. Absolutely disgusting.
Regards from a rather damp England.
Neil.