Hello Erik,
there are different points of view about features. Mine is purely personal and based on the personal experience of both radios. (I owned a Flex for 4 years)
I think that in standalone mode both are very similar, as you already sayd, but in SmartSDR, for example, it is not possible to change digital processing filter's settings like on EESDR2 (unfortunately I never used the version 3). Anyway in a high end radio I would like to do it.
Many of us (Flex owners) during several years, found both NB and NR not very effective while the EESDR experience was been much better.
In remote use, the one I most like, the radios are quite different. Smartlink doesn't offer a real advantages other than simplify access to your radio without a home (tricky) VPN. Furthermore, until version 2.4.9, the last one I used, I/Q stream was not supported, therefore a VPN was required. Then, enabling the I/Q stream, the network usage was comparable to that of any other SDR. I did a
test about it and also in this aspect no tangible benefit.
On remote mode, moreover, it is not possible to use a telegraphic key unless you have Maestro, while in EESDR it is possible just connecting it to one of the pc serial ports. In addition Maestro does not eliminate the needs of a computer (for the local cw skimmer and for the station log, just for example). With EESDR, all you need is a computer, a key or microphone and, if you like, the E-Coder. This makes a big difference in terms of usability.
There are other features related with the way software is designed. With Flex you need a different client for each platform but Linux is still unsupported, while with EESDR you just choose the right version for your os.
Lastly EESDR tries to get rid of serial ports and virtual audio cables through the TCI protocol, something that is not possible with SmartSDR. In the past, DAX drivers caused great issues (I still remember the Win10 - 2018 Spring update when all the radios became unusable).
On the integration side, the MB1 has its own serial port and it is enough for any needs. Actually it is the embedded pc serial port. On the others Expert rtx, this option is missing (of course). In the EXT-CTRL connector there is the RS-485 port which is an advanced version of the RS232, but at the moment it is not working yet. With it, it should be possible to implement a control system for external peripherals without having to go trough the computer.
While the TCI port is an excellent alternative it has the drawback that you need the IP address of the computer on which EESDR2 is running. A self-announcing system (which is not present) would allow connection to the TCI endpoint without having to know the IP address (I asked for it some time ago on the forum and I hope one day it will be considered
eesdr.com/en/forum-en / software-ss2 / 7748-how-to-get-ip-address-of-the-running-expertsdr2-pc-using-tci-protocol).
I hope this can clarify my point of view. Consider also, my English language skill is far away from perfection.
73' Enzo
iw7dmh