I'm not an amp engineer so I'll just spitball for you.
Impedance is resistance. The lower you go in impedance, the closer you get to a dead short. Ever accidentally (or on purpose) connect the two terminals of your battery together?
Think of your speaker load as a floodgate. The lower you wire, the more open that gate is and the more flow it's going to allow/pull. Further stipulate that your amp internals are like a pipe. That pipe is rated for 100psi. Depending on how stout your amp is, 1 ohm nominal could be like 50psi or 90psi. If you open that floodgate by lowering the load, you might exceed that rating and burst that pipe or pop your amp.
Boards, components, pipes, whatever... they're rated for a certain operating range. If you exceed that operating range, bad things can happen.
Lower loads also lower the efficiency of your amp. Those efficiency losses are lost in heat (mainly). More losses, more heat, more bad things. Components swell and contract, or just swell and pop (smoke show).
The better the amp, the more overbuilt it will be. .5 might be a walk in the park. Or it could be killing it. The components on the board and the board itself can only handle so much. Like I said, they are rated for an operating range. More of those components or better/bigger ones will give you more useable range. Lesser boards or components will obviously give you less.
This is what cracks me up about people comparing boards of different amps. They don't take into account the bits and pieces that comprise the whole. Or even the board itself. Just because it's the same layout doesn't mean it's the same weight PCB or the same size traces.
With all of that said, half ohm burps or 30 sec music runs should be fine on any amp worth running if you can hold the voltage. Low voltage is a whole 'nother can of worms. But if your amp is up to snuff... the protection should be good enough to keep you from hurting it too badly.
