This is one of the least stressful of horticultural industries. The stress of incoming products and material is about as exciting as it would be for any other retail store, but should not be too unbearable. Business management can likewise be stressful, but is no worse than the management of any other business. Being observant of plant material that may be in need of watering is actually not much more work than tending to produce in a grocery store. Yes, there are the occasional nasty customers, but you know where the shovels are, and how to dig a shallow grave. And how to plant some hydrangea on top of them so no one will want to disturb it.
Unlike grocery stores or other retail outlets, certain groups of plants can get infested with insects. Frosts can be devastating to unprotected sensitive plants. Snow that is not deep enough to keep the nursery closed for the day is probably enough to keep customers from coming in, and can make it unpleasant to work outside. Making snow angels gets old after a while.
Regardless, many people take retail nursery jobs because they do not want to deal with the stress of other work. Or when the last of their freelance catering gigs has dried up.