Maphead does it to me all the time. Turkey gives you a direct route into Russia without having to use transports or go the long way. Attacking Spain gives you a route to attack Gibraltar over land. Killing infantry alone with air power is really easy. In G42(tournament rules), attacking the neutrals at the end of the game can give you extra crucial economic victory income.
Unless somehow the allies know you are going to do it, they won’t necessarily be well positioned to get their gravy territories in South America and Saudi, and even when they do, the infantry may be far from the front.
The key is that you have to attack all 3 of Turkey, Sweden and Spain simultaneously. The other neutrals aren’t that significant.
The only problem is that it doesn’t do much directly advance your victory by getting key cities, victory chits, much more income or by hurting the allies.