There is some probability I have to move to another flat, for about 3 months, and there are serious problems with electricity. The lighting flickers frequently, and sometimes even worse: the electricity can even completely(?) disappear for a second. Here is how it feels: you work at evening on your stationary computer, and for a second the room is completely dark. Sometimes the stationary computer may continue to work after this, but sometimes it turns off.
Is it safe to take my M1 and M4 MacBook Pro laptops there? I don’t mean things like fire. What I mean is whether it is safe for their batteries, SSDs, motherboards and other details.
I work from home, and when I’m at home, my laptops are always plugged in.
No, your Macs are not safe under those conditions. A laptop with a battery should isolate the rest of the device, but can’t be good for your Mac’s battery.
Buy an inexpensive battery backed up UPS, something like this, Amazon.com: Tripp Lite Battery Backup Uninterruptible Power Supply That should isolate your Macs from brownouts, blackouts, and voltage surges. And if the UPS has a shortened lifespan, that is better than your Mac dying young.
As @karlnyhus says, it could cause problems. A sudden energy spike could fry your PSU and maybe also the motherboard/logic board, plus any other equipment could be susceptible e.g. monitors, docks, external powered hard drives or powered USB hubs. The cheaper your tech, the more likely it is to be affected.
At an absolute minimum I’d buy a surge protector, but agree with my learned friend that a small UPS from a reliable manufacturer is better. It will smooth out any electricity spikes (like a surge protector) but also maintain power from the battery in the event of an outage. You’ll need to size it for the equipment you wish to connect to it.
The more equipment you add to the UPS, the shorter the battery lasts while the power is out, so don’t randomly connect everything. Pick and choose.
There will also be a load limit on the UPS on how much power can be drawn, if you under spec it and try to draw too much, it will probably trip and shut everything down.