I see why running most lamps in horizontal position creates a greater temp difference in the arc tube (metal halide, specifically). But is it bad to go the other way around? What sort of problems would that create? I'm talking of course about the lamps that don't have any strange electrode arrangement.

Thanks

Generally operating high pressure lamp in other position (or outside the position range) then specified in the datasheet always lead to some problems. Remember, then manufacturers want to rate their lamps as universal as possible, so restricting some burning positions have always it's good reason in the lamp design or characteristics...

In vertical burning position one electrode is on top of the other one, what may create an overheating problem on the top electrode. E.g. on e.g. lamp like this the burning position with one electrode above the other one is explicitly prohibited, due to the electrode overheating problem...

Would you say in general for example that a BU-only lamp running BU will have a longer lifetime than a universal lamp running BU? And what about horizontal-rated lamp running horizontally versus a universal lamp running horizontally?

Lastly, is it worse to run a horizontal lamp BU, or to run a BU lamp horizontally?

Thanks

This is impossible to say generally. And my guess, it will be type by type different.
It depend on the exact reason, why the position was restricted:
It may e.g. not srtike at all, as the mercury droplet may short out starting electrode (this specific problem was the case for MA lamps, when BU type operated BD and vice versa)...
Or the lamp may fall off it's holder.
Or the socket may be damaged by the heat.
Or the lamp arc would be off-focus (in the case of "prefocused" lamps for optical systems)
The one electrode overheating may lead to lamp rectification and ballast damage
The overheated arctube may lead to it's explosion
And many more effects may be the reason for restricting the operating position...

With SOX or LPS lamps operation vertically cap down will result in a very short life because the sodium gets behind the cathodes and damages the seals. You can run the lower wattage (55W and below) lamps vertically cap up with no problems but larger lamps suffer from sodium migration and efficacy drops.

Edit: I've also heard that T5 HE and HO need to be burned etch end down if they're in a vertical installation because the etched end always has the cold finger in it. If you operate them etch end up the cold finger overheats, mercury pressure rises and efficacy drops

With T5 HE and HO, In Philips and Osram T5s, one electrode have a longer support then the another electrode, which make this electrode farther from the end then the other one. This is the cold spot of the lamp.
Most of the other T5s HO and HE have the same distances from the ends to both electrodes, resulting in a cold spot in the center of the tube like the T8s.