"Universal" supplies are just that
Utility power available to general consumers and travellers is going to be in the 85-265VAC/47-63Hz range that your average "universal input" SMPS is rated for. Reputable (read: listed) supplies are tested to every corner of this range (although some are only rated to 90VAC min).
Not having a grid isn't a problem either
Modified square waves (+peak, 0, -peak, 0, +peak) shouldn't be an issue -- the only potential problem is with active Power Factor Correction circuits misbehaving in mild ways (such as not providing proper PFC, or emitting undesired audible noises). (Keep in mind that just about all consumer standby power supplies and many low-end inverters generate MSW output.) Likewise, wandering frequency (from a generator) won't harm the supply much if at all -- the PFC circuit generally has enough bandwidth to track a source that drifts.
Noise filtering is a necessity on SMPSes anyway, so they generally reject external noise. Spikes are sometimes suppressed by a metal-oxide varistor or MOV (same thing you'd find in a surge strip), but are probably the greatest hazard unstable power can pose to a modern SMPS.
Weird and wild power sources
DC, interestingly enough, provided the voltage is reasonable (locomotive auxiliary ~70VDC is pushing it, but say a 115VDC or even a 380VDC rail should be OK), isn't an issue. In this case, the PFC circuit, if present, simply acts as a straight boost converter, and the input diodes do (almost) nothing.
Likewise, 400Hz aircraft/marine power sources won't cause much trouble either, although supplies listed for such use are generally only seen on more specialized gear, such as test equipment. Most input diodes can deal with it, and most PFC circuits have the bandwidth to roughly follow a 400Hz sine, albeit with degraded performance.
This leaves one more oddball to cover, and it hails not from some out of the way place, but North American industry. It's the 277V/60Hz single phase power sometimes found there, and it probably comes the closest you'll ever see to thwarting the universal-ness of your supply -- a reputable universal supply will start up and run on it, but with reduced margin for surges as the peak DC bus voltage is around 390V vs 340V for 240V mains in. Thankfully, 277V is rare to see at a receptacle even in large buildings in the US, and you wouldn't be able to plug a normal North American NEMA 5 plug into it anyway! (277V single phase uses the NEMA 7 receptacle.)