Had the misfortune of getting to test the X1C’s power outage recovery yesterday. 8 hours into a 10 hour print my power blinked due to a storm passing through. On recovery the printer did an x-y home and started laying down filament in what appeared to be the correct position, so I went about my business. An hour later I came back to check on it again and discovered that the entire print was shifted probably a mm on the x axis. Apparently this is pretty common. I can understand “home” being different by maybe a single step or two from one instance to the next due to mechanical variables like belt tension and thermal effects on the dimensions of components, but this seemed to be off by a number of steps. I guess it must be more difficult to accomplish than it would seem. Anyway, needless to say, I’ll be ordering a UPS today. And another roll of filament because I didn’t buy enough to allow for throwing most of one roll away 
In the few reports of prints stopping during power outages I don’t think I’ve seen any successful recoveries. Maybe they are there but the printer would need to encode absolute head position and also have a power supply that can warn the CPU that power has dropped out and then enough reserve to supply power while the state is saved. At least in the reports I’ve seen, the head offsets a little.
I run mine on a UPS that I got as an emergency power source. It’s saved a number of big prints now as we’re in monsoon season and lightning storms roll through frequently this time of year. I just unplug the UPS and computer and the printer safely carries on.
They need to be somewhat beefy but it’s so nice not having to worry about drops and blips on the line. To get the most bang for the buck the UPS doesn’t need to be huge since most outages are very short - but long enough to kill a print.
I did once have a brief power outage during a print and was pleasantly surprised that the machine did recover and the print was acceptable, but I would consider that a rare unexpected success and normally assume the print was going to be a throw away.
But at the end you mention not having enough filament left on the roll to do it over. With the AMS unit you have to be a little careful to set it up correctly, but the machine will do a good job of using up what’s left on the first spool and then switching to a backup.
I meant I don’t possess enough filament to complete the job. This part requires just over half a roll and I don’t have that much left on this roll, and no other roll of this same colored PLA to load into the AMS and automatically switch to. The automatic switch feature is nice though, except I’m always afraid that the feeder motor is not going to be able to pull the end of the filament out of the spool, and usually wind up standing there with the AMS lid open when it’s about to run out, and snip the filament just as it’s about to try to pull the end out of the spool. 
Unless the end is taped or tucked under something it should pull through with no issue.
I’ve used resume only once when the power went out. I have a UPS and just let it run out. Left it be and once the power came on it was fine. Guess I’m lucky.
I haven’t seen any spools fastened with tape at the end, but I’ve heard they’re out there - that’s stupid. But on some spools where the end is bent into the center I’ll reach in with narrow clippers and snip them. I agree just having a spool with enough filament on it is the most conservative approach in an activity with so many other risks that the conservative approach is best.
Nah, I use the auto switch all the time. Just printed a model that took a week, and 6 spools. A lot of time they ran out at night. I feel your pain, sucks.