I can tell you about our experience in two scenarios:
1) One of our employees has a 5ghz epmp sm connected to a ubnt Nanostation M5 that serves our office neighbors and the employee across the street. His stay at home wife is a heavy user, they have no cable and stream constantly. He games, streams, browses, etc. every night and no complaints or issues.
2) We had a tower get hit with a surge, took out 2/3 of the equipment on the tower, all ubnt, replaced all 3 2.4 with sync epmp. Replaced the 1 5g ap that got hit with epmp sync.
There was a total of 33 split between the 3 2.4 sectors and 17 on the 5g.
The 2.4 in wifi mode was terrible, would wireless drop clients randomly, could make any change in the gui and save it and they'd come back, work for a while, and repeat. After turning the RTS/CTS off in the ap, they got a little more stable, would stay connected for 10-24hrs. but would still disconnect and need need a change effected on the AP to get them to reconnect. This cost me many hours of sleep as I had to setup monitoring to alert us of the problem so that we could get customers back online as quickly as possible.
We changed all the 2.4 customers and the enabled tdd w/gps sync. The difference is amazing and the clients are all connected and happy now.
The customers on the 5g epmp in wifi mode seem to have no issues and have been chugging a long for over a month, similar results as the employee's home connection.
This leads me to believe that perhaps the AP is hearing too much interference in 2.4 and the 5g does not, but this is just a slightly educated guess.
SO here's what you need to know if you're going to mix them with ubnt and maybe other vendors, the ubnt gear has to be in wds mode and have airmax disabled, also if you're using channel shifting you have to disable that too. We found that disabling the epmp AP cts/rts thresholds helped, but ultimately until we changed all the gear we still were seeing issues.
In our bench tests epmp 2.4 SM connected to ubnt ap's had better results than epmp ap's with ubnt sm in wifi mode. This was lab versus real world though. Keep in mind that wifi mode was added as a second thought to the epmp line to help operators ease the transistion from other vendors, it's kinda of like making a ferrari a pizza delivery vehicle, it wants to race the indy 500 but is held back by the task you've assigned to it.
My suggestion is to wait until you can do it in a controlled manner, add the epmp as a second ap and migrate as you can, stock up the gear as the budget allows and change the cpe first, the tower upgrade is cheap (not accounting for tower climber, etc. depending on your situation), changing out all the cpe's cost more and takes more time.
If you're going to make the commitment to switch vendors, get some gear in and start using the epmp sm in wifi mode talking to the ubnt ap, NOW. For every new customer you install with a ubnt, is one more you have to replace later.
As to throughput for a mixed sector, we saw less than 5-10Mbps compared to the same 2.4 sector running 20-30Mbps now in all epmp and tdd mode. The 5g sector seems to run around 20Mbps too. Most customers have 5M or less packages on this tower.
Good luck!
Best Regards,
Josh Rowe