I hope others chime in with their experiences with these printer models, but I can give you feedback based on my experience with our 7900, 4900 (both died), and current 9900, as well as my perspective from answering support questions for our customers using these and other model printers.
We got the 7900 when they first came out a few years ago. The Epson techs had to come out three times in the first month of it's life to get it working right- they replaced the print head (TWICE!), ink selector unit (damper unit), cleaning unit (capping station, wiper and flushing box unit), and even the main board. It worked well after that for about two years (there were usually missing nozzles each morning, but they cleaned up after a few regular cleaning cycles, then it printed well all day). After about two years of regular use and decent results, the light cyan channel suddenly stopped printing the middle section. I did everything I could, starting with cleaning the capping stations, wiper blade, flushing box and bottom of the print head, as well as power clean cycles and installing PiezoFlush carts then doing an initial fill cycle- but the results were the same. I then replaced the dampers and flushed the print head (I actually tried cleaning the print head several times by taking it out, and carefully flushing it with PiezoFlush- which seemed to do the trick, since all the LC nozzles were firing after a few flushes), but after putting the printer back together and doing a few cleaning cycles/nozzle checks- the results were actually worse than before. I researched online for cleaning/repair tips, and found http://myx900.com/, which offers lots of helpful (and funny) information/videos about working with these printer models, including dissection and microscopic views of the print head- which made me realize the print heads in these models don't respond well to flushing like previous model heads, so we ended up trashing that printer (though I did learn a lot while working on it).
We got a 4900 shortly after they came out, and initially tested it with the Epson carts to learn about the printer (as we do with every printer we get). After a few months, it started having clogging/flow issues, and Epson sent us a replacement printer. We used the new printer with Epson carts for a while to make sure it worked well, then installed Ink Thrift dye ink and our refillable carts to test the ink and carts, and make some profiles for this printer/ink combination. After testing/profiling and having pretty good results, we had to move onto other projects, so flushed the printer and let it sit for a few months. When we went to use it again, the nozzle check was missing sections, and got worse after every cleaning cycle until nothing was printing at all. I thought maybe it was because that printer was not in a room where we normally control humidity, and it had been very dry, so set up a humidifier in the room, and continued cleaning/working on the printer. Of course by the time I called Epson for service, it was a few days past the warranty period based on the purchase date of the first 4900, so the were no help other than to say we could pay for repair. Based on our experience with the first printer, and all the reports we've received from other 4900 users struggling with constant problems, we decided it wasn't worth spending $ to repair, so trashed that printer too...
Our 9900 is newer (we did get the extended warranty on this one), and has been working pretty well as a production printer in our studio (with our refillable carts and ConeColor ink), but like the 7900, often has missing nozzles in the morning, though they usually clean up after one regular cleaning cycle, then it prints well for the rest of the day. After our experiences and overwhelming amount of problem reports we've received from other users (and what we've read online from when others have posted about their experiences with these models), we constantly cross our fingers and hope it works every day, not knowing if/when it will suddenly quit (which is pretty uncomfortable). As with all the production printers in our studio, we carefully maintain the humidity and temperature levels, as well as clean and use printers regularly to get the best results and longest life. I don't believe we'll get any more x900 or x890 printers when this one goes...
The x890 and x900 printers are very similar, and use the same print head technology and ink systems. The 7890/9890 models are essentially just 7900/9900 models without the orange and green channels.
When people ask for my opinion on what pro model Epson printer they should get, I can't honestly recommend the x890 or x900 without explaining what I said above, and tell them if they get one of these models, to protect themselves by getting the extended warranty. My personal opinion after working with a wide range of Epson printers over the years, is I LOVE the 7880/9880 models, and wish Epson didn't discontinue them... they're solid, consistently work well (we have 6 in our studio, and all have been working fabulously for over 5 years with very little maintenance), can be worked on and cleaned as needed- overall are very reliable, and produce excellent print quality. We wish we could get a few more, but they're not easy to find, and people who have them don't want to get rid of them.
So, I know this isn't super helpful as far as helping you decide which printer to buy, but is my feedback/experience.