30 plus cruises I only tried a guaranty three times. On a transatlantic Celebrity cruise (where I am an Elite) - did not get an upgrade even though there were many empty suites available. On an NCL cruise (my first with them) - got a three category upgrade to a nice suite. Both times I booked a cabin category that I would be OK with (even if I got the worst location in the category) and I use a very well known and well liked cruise-only travel agent. On RCCL got a two category upgrade on my first cruise with them (before they bought Celebrity).

(On the Celebrity cruise I happened to be invited to the Captain's table for dinner on the second night. We hit it off so at the end of dinner I told him that I had gambled on the upgrade and it didn't pay off (the price of the guaranty was the same as the cabin category - sometimes it is less). I kind of joked about it with him; the next day the Hotel Director called and said that Captain has a nicer cabin for me. . I moved.)

Some people think that upgrades go to first time cruisers with a line, to hook them. I don't know if it is true, although it was true with me.