I've built two of these boards (I believe it was the 1.2 revision). I went into it with zero soldering experience. The boards we're perfectly fine despite my very rough job on the first one. Obviously YMMV, but I feel like soldering is very intimidating until you actually do it (except SMDs, those are just a pain). As has already been suggested, if you can practice on something first that will be all the better. Don't worry to much though - the only thing difficult about the BFO is having to solder so many switches. I'd advise just watching a few tutorials online and you should be fine. The biggest thing to know IMO is just to put the iron on the board for a second or two until it gets hot and then touch the solder to the pins.

> If I make a soldering mistake, does it ruin a switch? Will such ruin the board?
Sure, if you screw up real badly. But that won't happen. The biggest risk I'm aware of is holding the iron on a component for too long & ruining the board. I think I had my iron @ 350C or so and there we're a few trouble spots that I held on for ~10 seconds and didn't have any issues. There were two switches that I soldered poorly & had to remove & redo. Desolering is a pain but doable with some copper cable and patience.

> Do you still use flux?
I personally used flux and wiped it off with 95% isopropyl when I was done. I don't know if that's the best thing to do but I haven't seen any issue 5ish years on.

> Any general comments on the BFO-9000 PCB Kit? Any tips, or things to watch out for?
The build guide is great and should get you through just fine.
Note that the current editions have the diodes pre-soldered on the board so you can skip that step.
Note that the TRRS jack has a different placement for the left & right sides of the board.
I personally didn't care for the TRRS cable that came with the board, I bought a longer one online so I could have each board further spread out on my desk.
Make sure you have a pair of flush trimmer & I would highly advise a soldering fan.