I read a recommendation that espalier should be performed with trees grafted onto dwarf roots.
What challenges or problems can I expect if I don't use a dwarf tree?
If it makes a difference, I'm considering apples and pears.
Apples and pears are very similar, so that makes this answer much simpler. You want a tree with moderated growth, because otherwise you end up with huge amounts of growth and very little fruit. You have a certain amount of space. A standard tree may fill the space quickly, but then it will put out dozens of long watersprouts all over each year, and won't grow budwood. Apples and Pears bear fruit on spurs, which are the result of slow, steady growth on a maturing tree, and the production of these spurs gets better as the tree gets older, and require pruning techniques that do not promote rapid growth.
And because most espaliers are to make production and maintenance/harvesting easier (as well as aestheticity), they are usually kept within human reach, sometimes with the use of a small stepladder. This calls for a tree that matures small. Bud 9 is on the larger end of dwarfing stock, and M27 is on the smaller end. If you can choose a stock to go with your setup, that's best.
It's better to use a tree that matures smaller than the required area, rather than larger, production wise. Another aspect is this: many dwarfing stocks have a non-aggressive root system, and so need staking even when grown naturally. These are better for use along walls, where trees that grow larger can hurt foundations. Another consideration is trunk diameter. A tree that maxes at 6-8" in diameter will be much easier to support, and less likely to damage the support than one that maxes at over 1.5' in diameter (I have seen much wider than that, but not on an espalier).
Another useful feature that dwarfing rootstock promotes is far closer leaf spacing, which makes for a much more exact branch placement. This can be the difference between a trim, symmetrical piece of art and a messy looking flat tree.
I suppose it might depend on the scale of the desired espailer.
Mind you, I have managed to notice that the drawings of espailered trees always look much nicer than the actual thing, in most cases. But I haven't pulled myself together far enough to make any of the things in 30-odd years of gardening, despite finding them attractive in books.
I would guess one aspect of working with non-dwarfing rootstock would be "excess vigor" expressed as a larger number of shoots. I suppose another might be refusing to fruit until a certain mass of tree is grown, which might be never if the thing is constantly pruned to a small size. Some of the "covering the whole end of a house" espailers seen in books might get big enough to cover that issue.
But I reiterate - NO practical experience here. I should find the time one of these years, but so far I have not.