The numbers that appear when a guide is in measurement mode appear above the circle that is used to control the guide. This can make it very hard to find the circle when the guide is covered by numbers. It would be helpful if the circle stayed visible by keeping it on top or making it larger.
Wow, measurement mode! The undocumented treasures of this app!
I think Georg adds them faster than he can document them 
I’m finding the measurements disappear when I turn the guide vertical.
I still lose measurements when I turn the guide vertical.
Also, like the OP I often find it hard to select guides, whether the little node is behind measurement numbers or I just don’t remember where on the guide to look for it. Would it be possible by double-clicking part of a guide to both select it and move the node to that spot? That would make moving them and working with them so much easier!
The measurement works fine for me if the guide is vertical (90°)
I will try to come up with something to improve the handling of guides.
Here's what I'm seeing. The only change between these screenshots is my rotating the guide to vertical.
Can you sent me the font, with at least this letter and the guide?
Perhaps an Option-dragged selection dragged over the guide's node should select it, like components can be selected.
(Though I would still like something like my double-clicking idea)