Anecdotally, I got one NOS from Ebay recently and had nothing but trouble. It seemed OK at first, but the 3.5" drive started having trouble reading the end of disks. Formatting them resulted in "bad sectors" that were not bad when reformatting on other drives. The 5.25" drive worked pretty well, but did have some problems reading at least one disk that another drive read fine (not terribly uncommon), and then crinkled the hub of one disk that I somehow(?) managed to insert incompletely before the clamp engaged. Still have no clue how that happened.
I took the 3.5" drive out and tried to replace it with another laptop floppy drive, but it didn't fit the mounting holes and faceplate, and didn't work anyway. So then I tried to remove the useless ribbon cable connecting the two drives so I could just use it as a 5.25" along with a normal 3.5" drive... and accidentally came in a little too low with the iron and melted the plastic shell around one of the head ribbon cable slots. The 5.25" stopped working after that, even though the ribbon could still be inserted. I briefly considered replacing the connector, but given all the other trouble I had with the thing, figured I would be better off without it.
So -- take it with a grain of salt. Some user error, lots of issues with what seemed to be a finicky piece of equipment... maybe I'm just unlucky, but I decided against pursuing another one. I'm using an Epson combo drive now that works pretty well.
On a related note, it seems Teac's later drives just did not hold up well. I have two late-model 3.5" drives -- one works.. okay. The other couldn't read any disk without errors no matter what I did. I have early Teacs, Sony, Mitsumi, and Alps drives that work fine. Also a Samsung that is obviously cost-reduced, and works... well enough.
Aside from that, 5.25 drives seem to be a little more fragile. I've lost two recently. An old Toshiba that I have some hope of fixing -- it worked for a while, but started acting up kind of out of the blue. Hoping I can clean its rails and have it returned to service. (Will have to take it apart to do so.) Another Mitsumi gave up after a recent earthquake. It was installed in my 386 desktop, which I was doing some work on, so I had it resting on top of a baby AT mini tower. Earthquake hit early that morning and knocked it off the tower. I found it sitting upside down on the floor -- having taken an NEC 84i SCSI CD drive along for the ride. The floppy drive refused to read disks anymore, but everything else survived with nothing but a few new dings and scratches on the cases. Not sure if that one's salvageable or not, but I kept it just in case.