No, that's not quite correct. They mfr give specs for 240 VAC and 28VDC because they are two common voltages, but the relay can be happily used for a wide range of voltages, AC and DC. (28 VDC: over-voltage on 2 x 12V batteries.)
Yes, there is a difference in how contacts work. These are silver oxide / cadmium oxide, and they are designed for heavy currents. There is ALWAYS arcing when contacts are opened, and that's why the contacts use those materials. It's a bit hard to 'burn' an oxide surface, after all. Other switches and relays use gold-plated sufaces instead: gold is good for LOW-level SIGNALS, but not for power applications.
One of the advantages of using plug-in relays (ones using a socket) is that if there is ever a problem, you can pull out the doubtful relay and replace it with a new one. That is a lot smarter than soldering onto the terminals of the relay.
'Shorter lifespan' - debatable. If the relay is rated at 28 VDC & 10 A, but you use it at 48 VDC and 1 A, will it have a longer or shorter life? If you have the spindle motor howling away at full load and you open the relay, there will be some arcing - although the relay is designed to handle that. On the other hand, if you kill the supply down to 2 VDC & 0.5 A before turning the motor off, how much arcing?
In this day and age of modern semiconductors, it turns out that power relay technology is pretty good. Even a wall switch works pretty well.
Cheers
Roger