Like all cars and their respective marques, there is always a problem child.
I can not offer any words of wisdom as far as the selespeed goes apart from the repairs I have had to do on customer's cars... Although!
The selespeed is generally a reliable piece of kit and lets not forget, it was truly groundbreaking in it's day... Just look at the amount of supercars that have flappy paddle gearboxes now!
All thanks to Ferrari and Alfa, the latter making it affordable for other marques to follow suit & even then the competition couldn't get it right for at least another 5 years afterwards
The selespeed got it spot on first time, it shows just how innovative Alfa really are/were in comparison.
The down sides are always the same, the more complicated you make a car, the more that can fail!
My work shop is simply jam packed with cars of all marques with electronic failures of some sort from ABS to ODB faults and yet very rarley with actual mechanical failures.
If in any doubt keep it simple, get a manual or alternatively get a 156 Q-System instead.
As you have owned a 166 previously no doubt you will be familiar with the many sportronics out there... Dont be confused into thinking that the Q-Sys is the same, it isn't!
Made by Aisin and not ZF, its a much, much stronger transmisssion that is fitted to Lexus/ Toyota and many Citroen PSA cars amongst others.
Lexus have the same gearbox running 400 bhp + without issues so it is more than capable of handling anything that a 2.5 V6 can chuck at it and it's simply bullet proof
So if you are in two minds with a selespeed, yet semi auto/ full auto appeals to you and you like the 156 shape and familiarity of the 166... That would be my choice, but i am biased really