The main issue that is visible for me in the SD quality is that deinterlacing is "slow", with the Azbox original firmware there is hardly any jagged interlacing lines even with fast moving picture. With the Vu+ Duo even with slow movement the intenlacing shows up.
There you're contradicting yourself
If deinterlacing is performed unconditionally and thoroughly, the picture maybe perceived as "slow" and "smeared out" during quick movements. There will be absolutely no jagged edges (comb effect) in that case whatsoever. If deinterlacing is performed dynamically, aka motion adaptive interlacing, interlacing is performed much more subtile, which prevents the smear-effect, but also can give a bit of a comb effect now and then. Please note that deinterlacing throws away a considerate amount of motion data, so it should be used as sparingly as possible.
On the same note, you shouldn't let your stb do any deinterlacing
at all. If the stb deinterlaces, all of the additional motion information is simply discarded. The TV should do any interlacing because it can make 50/60fps pseudo progressive from 50fps interlaced material and thus does not need to throw away any motion information.
If you configured your STB correctly, it would not deinterlace any material, but would apply "bob+weave" before scaling and "interleave" after scaling. You will find these terms on web pages describing deinterlacing (eg. 100fps.com), but technically none of them are. Never in this process any motion information is discarded.
What it does is: make full frames from every field, so the material becomes "pseudo" 50p. In this process some blending is performed to prevent (excessive) combing. The same considerations apply as with actual deinterlacing - tradeoff between fluid motion and possible combing - although no actual deinterlacing is performed! This pseudo 50p material is then scaled and afterwards interleaved, taking odd lines from odd frames and even lines from even frames, creating 50 fields per second from 50 frames per second, this is again 50i material and completely fluiently watchable on your television set.
As mentioned before, the "scaler sharpness" value on your enigma stb determines the tradeoff between sharpness and possible combing. I accept a little combing because it comes with a much sharper image. If you don't want to see any combing at all, you should decrease this setting at the cost of less sharpness.
On a related note, is there a difference in terms of different generations of Broadcom SoCs? I have understood that Vu+ Uno and Ultimo have more recent and powerful chip. Are you aware of any improvements or side by side comparisons bethween older/newer chips?
Afaik all current Broadcom SoC's have the same scaler hardware, only the dm800 has a SoC with a clearly lower quality scaler (according to some).