The very poor ergonomics of the traditional British BBb with mouthpiece too high and too far a reach to the 4th valve
The most ergonomically pleasing BBb bass I play is my own home practice instrument. A 700 series Besson. The mouthpiece is exactly at the right hight, the fourth valve is in a good position as the instrument is not too broad unlike a Courtois. It is easy to blow and it is a relatively light instrument (relative to the sov). The down side is that it has non-compensating valves and the sound projection is not as good as my bands' instrument which is a sov. However it is a joy to hold and play. I like the look of the new Neo, esp the positioning of the 4th valve but have never had a blow on one so I cannot compare the 700 with that particular beast.
I suggest that instrument designers could do worse than having a look at the ergonomics of the 700 series Bessons for some inspiration.