Our garden in Laguna was once an old avocado orchard and there are three remaining trees on the property. Avocado trees have notoriously shallow roots and you are not supposed to plant anything underneath them. They are also very messy at certain times of the year, shedding all their leaves when the new ones appear, providing their own mulch.
But these trees were at the very entrance to my garden and I needed to tie them in with the rest of the garden, so I ignored all advice and planted under and around them. That was fourteen years ago and if anything, I think they are happy to have all the company and attention. I don't advice anyone else doing it but it seems to work for me.
Under the biggest tree is an angel that has lived there for as long as I've been gardening. He has a tendency to get lost in the greenery and my husband is constantly clearing out the exuberant groundcovers and ferns that hide him. As you can see, even though he was fully exposed two weeks ago, everything is growing like crazy and he is once again hiding in the bushes.