Don't get extensions when you can actually REPLACE your cables.
You know Ohm's law, right? You're doubling the length of the cable. That increases the resistance and causes voltages to drop.
More often than not when we have users here, other forums, Reddit, etc. with problems of rebooting, lock ups, etc., it's because they're using extensions.
Can they be "dangerous"? Well, depends on who you buy from and what you specify. If you get 16g wire, you know they have to be using HCS terminals since the regular mini-fit Jr. terminals only support up to 18g. But then you don't know if the connectors used are UL 94 rated for flammability. If you look at a lot of mining PC's, you'll find it's not the wire that burns, but the connectors. The connectors actually get hotter than the terminals because the terminals are metal and can dissipate heat, but they dissipate that heat into the connectors which then store that heat because plastic is an insulator.
example: