One common application of "boom or bust" label is to imply that a player will likely need to hit something close to his ceiling in terms of potential in order to be an NHL player.
Usually, it is attached to smaller players, as they are often considered riskier prospects than larger players.
One reason behind that is the "modern" conceptualization of bottom six players as checking/defensive/energy players. Those are generally physical roles, and smaller players are usually at a physical disadvantage relative to larger ones. That means that larger players who don't develop to the point where they can be top six contributors have a distinct advantage over smaller players who fail to develop into top six players when it comes to filling bottom six roles. Hence, smaller players are generally more "boom" (top six quality) or "bust" (non-NHL career) than larger players who have are more likely to fall into that bottom six safety net.
Health is another reason that smaller players are considered to be riskier, as they are generally considered to be at a higher risk for injury in a contact sport than larger players.
The thing is, as the game has evolved over the last decade or so, those concerns have started to become less relevant. The crackdown on obstruction and some other rule changes has opened up the game, which in turn has made it easier for smaller players to succeed in top six roles and to find places in the bottom six (as more teams are now attempting to roll three scoring lines). As a result, we're seeing more and more teams reevaluate their philosophies on drafting and developing smaller places. Less emphasis on physical play and hitting is leveling the injury risk playing field somewhat. Thus, using "boom or bust" in that context is starting to lose its meaning somewhat.
Personally, I tend to use the term to describe a prospect with one or two top six/four quality skills/abilities in place who have significant flaws to overcome before they can be considered quality prospects. The opposite would be a "safer" prospect who doesn't really have any fatal holes in their game, but who also doesn't have any impact skills or abilities...low ceiling, "high" floor types.
