I wonder if a single bad bit could permanently destroy a whole ~16MiB sector? So SMR drives would need significantly more reserves to compensate for that.
So no, a single bad bit would only affect a single sector. And in fact, a single bad bit wouldn't do any harm, as all drives use error correcting codes that can handle a lot more than a single bit going bad. You'd actually lose a whole sector at time (512 bytes or 4096 bytes, depending on the drive).
This means that when writing data it overwrites neighboring tracks, and hence writing is partially overlapped like shingles.
Apart from that they're more or less the same as normal drives as far as I know, firmware aside. So a single bad bit shouldn't affect more than in a non-SMR drive.
I based my post primarily on what I recalled from this talk[1] by a HGST engineer.