Elkaholic, your gunsmith is misinformed, the #1 performs very well with the big cases and is preferred by many for the big cases. It comes standard in 416 Rigby. I have two of those. It is very strong and solid and for many years was the preffered action for many guys building wildcats like the 338-378 wby and 338-416 Rigby.
I agree with hot enough loads you could probably ruin about anything. The beauty of the #1 is it balances out so well with a 30" barrel and is still fairly light weight. With the 30" barrel it is still about the same length as a standard action with a 26" tube on it. Several years back I built several big 338's on the #1 and was able to work out the trigger and accuracy issues with them. They are still handy little rifles with the long barrels.