[prasanna moholkar]

Why MS not clear about low end Lumia windows 10 devices like Lumia 520 officially
As it is most popular device in WINDOWS phone history.

[RumoredNow]

I think it is pretty clear that 520 WILL get Win 10 in some version, but hardware limits may prevent some features from being added.

[prasanna moholkar]

Whether the preview update is full of buggy like MS pending project Live lock screen beta,tetra lock screen???

[dKp1977]

Whether the preview update is full of buggy like MS pending project Live lock screen beta,tetra lock screen???

There's certainly a lot of bugs to expect and functionality to be missing. That's why it's clearly called a preview and neither Microsoft officials nor tech sites are getting tired of trying to inform users about that fact.

[RumoredNow]

Why MS not clear about low end Lumia windows 10 devices like Lumia 520 officially
As it is most popular device in WINDOWS phone history.

Whether the preview update is full of buggy like MS pending project Live lock screen beta,tetra lock screen???

Bottom line?

It's not out. No one knows EXACTLY at this moment.

Don't rush into installing it the very second it becomes available - ESPECIALLY IF IT IS ON YOUR ONLY PHONE. Wait a bit, have a little patience and see what gets reported by actual users on actual phones using the actual Preview. Rumors are rumors and by their very nature full of speculation that may or may not be factual.

There is no factual information to provide... It's all theoretical and rumor based at this date and time.