I have searched and looked over many posts on performing a SAS, but i could not find an answer to this question:

Would there be a problem with doing a SAS using springs in the front and keeping the coils in the back?

I read this post in Pirate where they removed the coils in the back and replaced them with springs over the axles. I think it was to match the heights of the front springs, which were also over the axle.

What would be the draw back to doing a SAS and keeping the springs under the axle in the front and leaving the rear suspension alone?

You can't completely leave the rear alone. Even with the front SUA, you'll have to lift the rear to match... just not as much as if it were SOA. Doesn't surprise me that people go leaves in the rear during a SAS with a lot of lift. Probably easier from a design standpoint to swap out the coils for leaves versus redoing all of the links (the stock links won't be long enough for a large amount of lift). But, no, there is no harm in having coils on one end and leaves on the other.

many people do coil rear/leaf fronts because they gain the advantage of a linked setup without the complexity (and cost) of dealing with linking a steering axle.

Front linked suspensions typically can't run a dual triangulated 4 link which seems to be the strongest most versatile and effective setup you can use in the rear, due to engine oil pan clearance.

Instead you can run a 3 link plus panhard bar with the two lowers parallel but making all of those links work in harmony with your drivetrain and steering and strength requirements is more difficult.