I have in the past considered the unincorporated territory located outside official municipal boundaries that is addressed to or is located within the same ZIP code when reclassifying places. I think this can be criterial but not definitive to whether a place is or is not a village, town, city, etc.

Generally, when mapping populated places, I'd rather apply a place node to a community that has borders which are federally-recognized in some way rather than take the word from people in the area, because the way someone might describe where they're from or the experience of how they know a place as varies by person. I've heard people say they're from "DC" but that ends up being Arlington, Virginia. One might also say they're from Reno or Las Vegas but it's actually Sparks or Henderson.  I live in a CDP myself, and how I say where I am from varies between the specific CDP I am in, the place my home is addressed to, or the primary city of the metro area it is located in. If you simply follow the designation of a place intended to be equivalent to a place with a name, borders, and a population but no local government, then you avoid any COI. While I do think some semblance of local knowledge is important on OSM, I do think relying on a greater source similar to how Wikipedia does is too, at least for things like these.

As for place=suburb tagging, indicative by its European name and most common usage, it is intended be used for what would be simply a large neighborhood in the US rather than the informal American term "suburb" which means a myriad of things, like a smaller town nearby a larger city, a small residential subdivision, the outskirts of a city, etc. Sun Valley and Spanish Springs are functionally more akin to towns rather than neighborhoods within a city. Sun Valley even has its own municipal address, with Spanish Springs having an acceptable alternative city name. I think that makes them stand out a little more than as suburbs by OSM definitions. 

I also think that it isn't necessary for a community to have a central business district, though that does strengthen their independence from nearby cities. Sun Valley does have Sun Valley Boulevard which acts like a "downtown" even if it doesn't look like one. The corridor houses most of the area's commercial buildings as well as the Sun Valley Community Park and the post office. Spanish Springs also has a main commercial area at the intersection of La Posada and Eagle Canyon at Pyramid Way. These aren't the first places you'll think of when you imagine a proper downtown area, but many, if not most towns and cities that started developing after the mid-20th century when cars hit the mainstream look like this, especially throughout the Sun Belt region. Cape Coral, FL is a good example of this, where its downtown is mostly pad sites and strip malls in a city center-like grid.