Earlier quoted context omitted.
A power-only (2 wire) magnetic USB-C cable will be as safe as prior generation magnetic USB cables (it won't be more likely to damage your device)
Sure, it will be safe because it won't be able to go above 5V 3.0A (15W) at absolute max since it won't be negotiating PD. Most likely it won't have the proper termination resistors on the CC line and will get a whopping 900mA@5V = 4.5W * https://i.imgur.com/n3IjcQE.png I encourage everyone to actually read the USB 3.2/C/PD spec sheets.
Aside from that: It's obviously not enough for a laptop, but what other devices do you really need that much more for in your day-to-day life? I think I haven't used a USB charger capable of outputting more than 10W in a while (aside from my Nintendo Switch and its charging brick, but because of the dock it wouldn't make much sense to use a magnet adapter anyway). At home, there's always a charger nearby, and when I leave the house my phone usually has enough power anyways, or the trip is long enough for me to throw a powerbank in my backpack no matter how full the phone battery is.
And if you end up in a situation where you really need to quick-charge something, there's nothing stopping you from just taking the plug out for a short while.