As someone who's done far and away too many downlights for the 240v Au market, heat is your enemy.
Regarding regular halogen downlights before LEDs.
The downlights that have a separate transformer/driver should be, technically, in a holder. These have come in various forms. A metal cone was made so that the downlight gimble could still remain open at the top for 50w
GU5.3 base halogen globes. The iron core transformer usually just sat on the insulation but they always should have been placed on a square of fibro, away from any timber.
The metal cone was good because the 92mm hole could be drilled and the light fitted from underneath. When the equivalent, better, 35w halogen showed up, you could use a plastic box that needed to be attached on the roof side. It had a 92mm hole which lined up with the ceiling hole and also had a pocket for the newer solid state transformer so it was not sitting on the ceiling and had airflow. But they were a bastard to fit, for obvious reasons. Government jobs they considered mandatory and new house work they were 'supposed' to be.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
Luckily the low-watt LED replacement alleviated much of the waste heat and therefore the worry. The separate driver should be placed in the enclosure and they do make all in one 92mm driver LED with a cord. But those do fail early due to heat.
The GU10 240v incandescent light fittings were hit and miss. The rule of thumb, never put them outside (under the eaves). The globes ran so hot that if the fitting was sealed, it failed early or if unsealed so it could breathe, it attracted moisture which got into the cracked terminals and springs of the GU10. So the globe would blow and you'd be up for a new fitting.
Inside the house, though, they were fine. You'd find them used often in short track lighting with not too many problems. It depended on the condition and quality and plating of the terminals of the lamp and the fitting. And, again, how much heat.