It doesn’t take very many units because all you have to kill is infantry. It also doesn’t give the Allies very much money or troops, or at least not if you take all the neutrals out in the Euro Zone. They get like 3-4 2$ territories that take them time to liberate because you broke neutrality on the Axis turn they may not be positioned to do so. The best positioned territories that the allies get are SA with some soldiers and Afghanistan, with 4. Japan doesn’t have to deal with any but Mongolia and that is a separate case so it neither helps nor hurts Japan much to violate.
Just remember the big caveats here;
You cannot fly over the territory of a neutral until it has been violated.
You cannot blitz through a neutral with tanks but must stop and “put up flags” as the taker, even if your units had moves left.
You can land on allied territory, but not pro-allied until it is owned
If you fail to take the territory, you place units of a power there, but they cannot move or fight. They “become” or “belong” to the first friendly power to place a land unit there and so does the income. Remember, you could in this case land friendly planes there.
If Sweden is ever pro allies (as it would be during an Axis take of any neutral), the NO for Germany is lost. This one is often forgotten.