It is better to keep logic inside of the HUD because the code is directly related to the HUD. If it were in the game-mode blueprint, I don't think it would even work. Variables don't seem to cross over blueprint classes. As well, keeping code tidy is important. Navigation in large projects is an important quality. Being able to know where some code is based entirely on what the code does and what blueprint class it would be most closely related to is a better system than, say, just going off the dome with locations.
I had two "major" (in quotes because neither took more than 10 minutes) problems. The first was figuring out how to create a configurable timer with only text and my wits. Luckily, I had a tutorial for that. Hooray! The second one, which also technically had a tutorial, was caused by my missing a step in the power-ups section of the first HUD tutorial. I didn't see the part where the director-person had made sure to configure the icon colour upon activation. As a result my icons became pure black squares upon triggering. I did find the section in the video, and I was able to fix it.