Plants grown in containers on any sort of long term strategy are always going to be more prone to cold damage than those grown in the ground. Roots are much more vulnerable to permanent damage from cold than is the top portion and in a container, lack the insulating properties of that vast soil mass surrounding them. Irreversable damage can occur at around 26F.

A 'lifetime' guarantee is virtually unheard of in the nursery industry. There are any number of natural causes that can result in the demise of trees and shrubs and it's just plain silly to guarantee against them. More so with J. maples than with most other species. But I guess it's their buck :-)

If you want to take advantage of the guarantee - and who wouldn't - plant in the ground.