You won't know till you get stuck in - it depends on the variety of Mahonia, how tall it is, how long it's been in the ground, and even with all that, there's no way of predicting quite what the roots have done or where they've gone, other than to say, if its a tall one, it will have 2 or 3 big roots going down into the ground, with a collection of smaller ones roundabout, some of which will be 'anchor' roots and may not be that far under the soil, growing horizontally.

I may have to put some Stump Kill on it Bamboo LOL

It should come out quite easy, however that’s easy for me to say as I am always grubbing out old large shrubs, and that’s the way to tackle it, with a sturdy grubber, start about two feet away and work round it, smashing through the main roots, shove an iron bar in there for leaveridge if need be, it should take no more than five mins doing it this way, just make sure you have no underground cables etc near to base.

My Dad used to use a scaffold pole, I forgot about it LOL, he would dig a hole & put the pole down it & use a rock as leverage, that’s how I got rid of my Bamboo LOL, Dah! Silly Old Me in forgetting that idea, must be my age LOL