FWIW, I had a pretty good apple bonsai I got from Chase Rosade 15 years ago. I loved it. It was awesome--until it wore me out.
I kept it for eight or nine years then had to sell it. It was a gigantic pain in the behind. It drew bugs of every description into my backyard, from aphids to wooly adelgids, to Japanese beetles and borers and everything in between. It was the only tree I've ever used a massive program of preventative insecticide spraying for, beginning in the spring and lasting into early winter.
In addition to the bugs (and expect A LOT of borer issues with landscape apple trunks cut down into bonsai--all that exposed fruitwood from the trunk chop is a candy store for them and they can smell it a half mile away--literally). Expect mold and fungus issues too, especially in summer. Also expect intense rodent interest in the autumn and winter. Winter storage has to be cold, but sheltered. That shelter can become a death trap for the tree if mice and other critters have access. They will chew off bark, twigs and branches and can wind up killing even large apples if they succeed in girdling the trunk when they're dining out. Deep snow cover can mean big trouble as mice will tunnel to your apple and set up shop.
Also, if it's not a prolific blooming crab cultivar, expect some pruning issues. It can take a bit of effort and correct timing to get blossoms on some breeds, especially regular orchard-type cultivars. You have to build up blossoming tissue in branches (and be able to recognize what that looks like).
All this is worth it--IF the tree you have is really nice -- and older apple bonsai are extremely picturesque. However, the effort required to keep them can become a real, REAL drag, especially if your tree is outright killed by a mouse one winter.
Toringo crab apple at the National Arboretum.