Second that. I, too, find the “prefab” predicates too restrictive and miss a more SQL-like language. “Select all tasks that match X, show them in a tags perspective, group by tag, filter by ‘Today’”. Not possible. And so forth. It’s a constant struggle to get your system to work for you.
Another example of restrictiveness: only one flag. Sometimes you need more, so as not to affect existing flags.
On the other hand, OF is too unrestrictive for novices! It’s difficult to get started and I’ve wondered for a long time why it doesn’t ship with a couple of “standard” setups out of the box to get people going. More users would benefit all of us.