I One big advantage with UM is the ease of changing nozzle. I like to change nozzle size depending on what I print to compromise between print quality, part strength and speed. I have had to repair my i3 MK3 twice from damage occurred from changing nozzles (once was due to grabbing the heater block the wrong way and braking one the thermistor leads. The other time the internal threads on the block stripped out). Other than that I love the e3d v6.
Using non UM filament on the S5 is no problem at all. You might not be able to use the included filament holder, but not having the filament in the back makes is much easier to change anyways. When using non UM filament you have to select the closest matching plastic from the menu and that's all, the rest you fix in Cura.
I have not tried ABS or ASA in the S5 since I have access to a Stratasys with heated chamber. The S5 should in theory do better than the TAZ Pro since it isn't as open. A optional enclosure will probably be available for the TAZ Pro at some point though.
If you plan to print flexible then bowden isn't a good idea. Changing filament is very slow on the S5. I haven't tried the TAZ Pro, but changing filament on the i3 MK3 is a lot faster than on the S5. If you often change filament this may be a factor.
The very stupid (IMHO) placement of the filament holder and extruder motors on the S5/S3 are something you should take into consideration. You have to be able to turn the printer around in order to change filaments. The alternative is as I mentioned to use a external filament holder, but the extruders are on opposite sides in the back so you then need to have access from both sides of the printer to feed filament. And if you don't have plenty of space on both sides you'll have to feed the filament blindly by touch. You also loose the NFC capabilities with custom filament holders. This is by far the most negative thing about the S5.
I can't think of more to say about the S5 off the top of my head, but I hope at least some of my opinions are of value.