I have booked guarantees before and I could choose the category but not the cabin. So I could choose a category that that has a window in a good area of the ship. They never put you in a lower category that has a porthole. So as long as you are happy with the category you choose, you won't be disappointed and may even get a pleasant surprise.(bolding by me)
I supposed there might be some "one week before the cruise" guarantees that are run of ship and they can put you ANYWHERE, but I don't think I would go for that.
That's an interesting thought - I have always wondered why most cruiselines just give you a balcony guarantee, a suite Gty, an inside Gty etc. but this can be a box of worms as well as a pleasurable surprise.
If with HAL you can choose the sub-categories you may at least get the minimum you're content with - like an Oceanview cabin with a real window. Seems like a compromise.
Personally I'd prefer to book a specific cabin but it's another of the HAL surprises - on some of the cruises it is not even possible to choose as ALL categories are Guarantees... And sometimes you're stuck with that even when booking a long time before the cruise if you are keen on a specific itinerary!