Data was explicitly said to be unique in the sense that he was the only one in existence. That is, as of the 2370s, there was but a single sentient android (or artificial ife form) in the entire Federation, and Data was it, according to Torres in VOY "Prototype".
This is different from Odo being a unique type of shapeshifter, in a universe teeming with shapeshifters of all sorts. It is also different from Odo or Data being the first of his sort - something never stated or suggested, except within very narrow additional parameters. Data was "the second fully functional Soong-type positronic android known to have been constructed", to type it out in full.
This in itself prompts certain speculation. If Data is unique, but not the first, then it appears likely that there simply is zero demand for his sort. Or possibly low odds of survival, considering he wasn't always unique, and others like him have either perished or gone into deep hiding.
Before Data and his ilk, the UFP had met at least Ruk, a fully sapient Data-analogue, and Norman, a two-eyed king to an admittedly blind android community. The underlying technologies were open to UFP study afterwards, not destroyed in a cataclysmic explosion or anything. But Soong's colleague and competitor Graves ridicules the concept of playing with android puppets, and a court is swayed by a straw man argument about tin man soldiers or slaves; clearly, mechanical men aren't a good thing a priori.
That Soong nevertheless built sapient androids might be because those were an exceptionally good marketing gimmick for his true achievement, the positronic computer. Classic optronics might outdo positronics in all other applications, but perhaps a compact AI brain inside a humanlike braincase would work best if positronic? Even then, Soong never dared make a public demonstration, fearing ridicule - but something good came out of it, as Dr Bashir later is able to use positronics in brain prosthetics, a feat perhaps less facilitated if Soong had chosen to work on positronic starships.
In general, lone inventors working on supertechnologies of dubious practical worth don't IMHO create an inconsistent and splintered fictional universe. Rather, they are a Trek thing - and these mad scientists failing to share findings or goals even with close colleagues is another feature, perfectly allowing for "unique" accomplishments even when there exists precedent of sorts.
Timo Saloniemi
