The ones I had a decade ago were a scam.
There needs to be some side growth-the fruiting comes on lateral growth. Smaller
apple trees tend to need support to hold the weight of the fruit. The columnar apples seem to have the "feature" that they can support the weight on an awkward top heavy trunk without toppling over or breaking the graft union. Decorative, but a prime objective? Even if they did produce (mine didn't), they aren't bred for flavor. Isn't that the point?
Do an internet search on "spindle
trellis systems." You can pack the trees in a meter apart, using dwarfing rootstock. You can graft pretty much any variety you want onto that. I saw in a quick search myself that the method has been adapted to cherries. Pears are a classic espalier choice, as are nectarines, so the spindle system
should work for them too.
If you are tight for space, I am guessing this is zone 1 and it needs to look pretty. Take a look at "Belgian Fence" espalier systems. You can pack trees in at 50-75 cm in that system if you want. I think they are beautiful, and eventually they can self-graft where the branches cross and not need support at all.