A single field also breaks the expectation that people do not get called by their full names in every interaction. This is a very common expectation, and violating it makes you sound subtly more like an evil robot.

Is this a lesser offense than mangling a name that doesn't cleanly split into first/last? At the individual scale, probably.

The impact, in aggregate, on UX/sales/utility? Could definitely go either way depending on your userbase.