Back when USB audio interfaces were buggy, FireWire was stable if you were willing to pay for the infrastructure around it.
My band has a 1220i, which we unboxed and then recorded an album that weekend. It worked perfectly, and continues to serve daily duties in my home studio.
We also have a 1640i, which we unboxed and recorded the start of an album over a long weekend. Worked great, and still works great. It is bulky though, and I challenge you to find the dang rotopod or rack mount kit anywhere on the Internet. These standalone kits seem to now be unobtanium.
I still use the 1640i for projects or large improv recording sessions.
Stability wise, in 2019, they work pretty well. As well or better than any USB interface I have, including the SoundCraft MTK Signature 12 and the k-mix. I need to reboot the soundcraft once a week. The kmix has a stuck rotary out of the box that I’m too lazy/busy to fix. I need to reboot the 1220i a few times a year, otherwise it stays on. I have it connected to a MacPro 5,1.
Now that USB interfaces and the chipsets used in audio devices are more reliable, I’m looking at an MTK 22 to possibly replace the 1640i, or maybe that 16 channel zoom. Or not. I will probably use them as long as I have a Mac that supports FireWire, or the mutant $100 worth of dongles that allows a thunderbolt 3 Mac to connect to them.
They also work great as live mixers, but there are no built in effects which is why I now use the MTK 12 for small live shows where the vocalists want a little reverb. The soundcraft also saves some desk space in an already small mixing space.