I have no first hand experience with Asko
All my girls are have Asko somewhere in the lines.
First one is Kessy von der Grube Waldecke, a grand daughter to Asko, and a grand daughter to Troll von der Boesen Nachbarschaft.
She was very tough and intelligent, independent. not too much prey drive but lots of defense drives, came to herself and peak in work with about 2- 3 yrs old. Tough and determined in all her doings. Though people who knew Troll and saw her- were more inclined to call her a "typical" Troll!!!
I have kept a few of her offspring, they all are tough and determined, combined with high intelligence. Her one daughter is a copy of her temperament wise ,but looks the total opposite. All offspring from her are intelligent and all capable of working hard and determined.
Two of my other dogs, go back to Asko son Yago von der Belferlein, one daughter of Yago, one Grand daughter. I like them, and all the dogs I have met out of Yago.
Hard workers and all intelligent.
As to the maturity thing, It may be just my point of view, but I call them late bloomers. Not because they are slow, no -exactly the opposite,because they are very intelligent.
The tendency for them to give it their all early on, makes them easy to train, but can easily get overload. They are busy in their heads, and it takes some time to get it all absorbed and put together, and this happens with maturity. Yes , they can be challenging to raise, but if you keep patient and keep educating them without overloading them- or asking too much too early ( beware because they are willing to give it) you will have a great dog once they come fully into their own.
How much of this is Asko, I don't know, but you may be able to paint a picture if you put all knowledge together and see what seems to be consistent?!?
There is was an artice in the SV magazine quite a while ago about Asko. I do not have the exact year and Edition, but maybe someone does and you care to find it and translate it... ?!?