Loki :
Are AUS8, 440c. ATS-34 really too brittle for use in a large blade.
The Stainless steels are more brittle than the tool steels which are more brittle than the plain carbon steels. If this is a functional difference depends greatly on how you are planning on using the knife.
It is not like even the more brittle stainless steels like ATS-34 are going to shatter to pieces when cutting grass, hay light weeds or even some light wood. The main problems will be with the impacts that happen on occasion that you wish did not. Chop into a piece of wood, hit a nail, cut some weeds, hit a rock, that kind of thing.
I have used ATS-34 heat treated by Bos (and others) and in such contacts it will fracture readily. The more durable tool steels will fracture less and tend to indent more, the tougher carbon steels will just indent.
There are some kinds of use that will cause similar damage, for example chopping on heavy bone, especially if seasoned, cutting heavy guage wire, trying to dig/chop through rooty soil.
The more durable steels will also be able to take very heavy impacts to the spine so you can for example hammer on them with very hard objects for splitting purposes.
In regards to custom work the optimal solution is to discuss what you need with the maker and he should be able to tell you if you will have a problem or not.
-Cliff