Rhododendron looks pretty but is an absolute plague in Ireland. It grows so big and vast that people sometimes get lost in them.
It looks nice on its own, resilient miniature of pines, resin produces very nice smell. No issue crossing regular pine forests. The problem is, these inhabit cca solid band in cca 1400-1800m altitude (as in, all mountains in the area normally have them, sometimes big blobs but often uninterrupted). Interestingly I've never saw them in the neighboring Alps.
When coming up close, you realize that either you have pre-cut way through, or you better turn back or find somehow way around. Few times some old hiking path disappeared into it (more like they grew over it), getting 200m through could be easily 1-2h solid effort that left me physically wasted. Also they are rough and destroy even sturdy outdoor clothing easily, and I always have various scratches. Resin will cover you from head to toe, usually not washable from clothes completely.
Suffice to say, I hate it when I encounter them off major hiking trails. Also bears love hiding in them.