From what I've seen on other threads, I think it's just that some people find them a bit boring... generic shiny heroes, compared to the richer themes of the other Exalted... shapeshifting beasts, reality-warping martial artists, cyborgs, etc. As I recall, this view was particularly common among Infernals fans, who saw that part of the setting as a fun trans-humanist playground... by contrast, Solars represented a certain conservatism, looking backward to a past golden age even as they upset the current order.
Suffice to say, I find anything that helps me make a Solar more strange or weird or more like another Exalt in my eyes quite interesting. I consider it a self-made challenge to figure out why a Solar would end up imitating another Exalt intentionally or unintentionally, how they would do that, and what tools to give them that best allows them to do this. Infernal-esque Solar is a bit easier to achieve than you might think-it involves being demon-blooded and being a huge sorcery nerd while fluffing all your spells to be malfeas-like, though some mutations would probably help. Abyssals and Dragon-Blooded just involve having the right material artifacts and the ability to fake their animas and being ghost-blooded for the Abyssal, Lunars involves being a beastman beforehand then uses Flawlessly Impenetrable Disguise to be in your "human" form, some moonsilver, though some Tiger Style helps. Sidereals are actually the hardest: you need to Supernal Lore to get future prediction, sorcery to summon elementals and some occult charms to follow up, flawlessly impenetrable disguise and then learn martial arts.
Alchemicals merely requires that you figure out a way to have metal skin and a gem on your forehead then take artifacts to match their color. how a person knows about robots or an Autochthon-like being to imitate one merely requires a strange obscure cult very interested in lost wonders. Sorcery that fluffs the spells in a mechanical manner helps.
the REAL hard part is figuring out the actual backstory that leads to these people existing. it takes some pre-ordaining and choosing the right environment.