First part - http://www.personal-view.com/talks/discussion/12604/smart-watches-apple-sony-and-samsung-doing-strange-things#Item_2

It is important to note one thing considering smart watches. Most of them need functioning Bluetooth 4.0 LE.

And even if you phone has Bluetooth 4.0 in the specs it does not mean anything.

• For Android OS. You need stable firmware for your phone based on Android 4.4.4 or later (5.0 and up is fine). Early versions have various issues, but can work sometimes.
• Preferably your phone must not be based on MTK chipsets (around 98% of Chinese phones). Many watches have issues with MTK chipsets.
• You can have issues if phone has other brand sometimes (some functions won't work even after hacks on non Samsung phones for Gear watches).
• Phones using Windows Phone OS are no go for everything except Microsoft Band (and it is hardly smart watch).
• iPhones will work only with iWatch and some Chinese smart watches,

One of the best phones for watches now is older Galaxy S3 and S4, both have updated firmware and work ok with most watches.