"a lot of people try to make as to why Apple won't allow third party watch face development"
A lot of people may say this, doesn't mean it's true.
Much more likely, I suspect, is Apple concerns over power (and perhaps also memory). Giving an app access to the watch face is giving it a huge opportunity to burn energy constantly, and we KNOW that developers abuse that sort of power, and that Apple then gets blamed. (Consider, eg, how two high profile apps --- Skype, and Facebook --- both burned large amounts of battery, which made many iPhones, and Apple, look bad. And that's from companies that are large enough that they should know what they are doing.)
Every time developers screw up, Apple tightens the screws and tries to ensure that their next round of products aren't as susceptible to that particular abuse.
Temperature, I think, can be handled well through a bolometer. Many "sports" watches (diving, climbing, running) feature temperature sensors, and given what they cost and who uses them, I assume those sensors do their job well.