I don't think it's that planks are easier. It's that...
1) Most of us learn on the R22, which is the most difficult helicopter to fly...probably the twitchiest, most sensitive, most likely to get out of control flying machine there is actually, much as I love them.
2) Helicopter PPLs normally want to land in friends' gardens and hotels and suchlike, and need to be safe to do so. This is probably akin to your new PPL(A) wanting to fly to Lundy.
3) Autorotations are not more difficult than f/w forced landings, but you need faster reactions, and those reactions need to be instinctive. So they are practised more. In the PPL(H), there is a basic auto lesson, then advanced autos, then forced landings. I don't think it's that many officially in f/w, is it?
4) Instructors worry more about letting low hours helicopter pilots loose, especially if they fly R22s. I'm not sure why; maybe there are more things that can go wrong since they're more complicated machines. But I remember a discussion in the USA, with instructors saying the course should be offically 60-70 hours.
5) Generally, with an aeroplane, you've got to get it in to the air before you can make mistakes. If you're a bit rusty, you'll probably come a cropper on the landing.
With a helicopter, you can get dynamic rollover during take-of, and the machine will probably thrash itself to bits. Hover-taxiiing requires well practised footwork, especially in a stong wind.
6) Related to (5), accidents on the ground or at low speed usually only bend aeroplanes a little bit. Helicopters have a horrible habit of thrashing themselves to bits if they turn over. This gets expensive if it happens often!
I still say, having learned to fly both, helicopters aren't more difficult. They're just....different.