I love the concept of Collections, I use them a lot. Basically anything that could be grouped around a specific theme, project, verb, etc. ends up in a Collection. But there’s one thing I don’t like about them and that is that they’re mixed in your daily logs.
I want to keep an overview… my index becomes too messy when I have many page number entries for say April, because each April-page is followed by some Collection. I considered this a weak part of the Bullet Journal method.