"But I suppose it's OK to share on personal pages designs (winning or non winning) from contests WITHOUT NDA, or not?"
...winning entries, they own the copyright, that's part of the transfer agreement, so you cannot use it, in any way, without first getting their permission.
...point is, once you've signed the copyright transfer then you no longer have any rights to the design, hence why you should ask for explicit permission to use them, even in a personal portfolio.
As far as non-winning designs, even those under NDA, I'm of the firm opinion that if you strip any identifying elements that can tie it back to the original project then you should be alright. The point is to not cross the line of of being visually similar enough that it can be identified as originating from that project.
Hypothetical situation, say you did a design for a restaurant called "The Elephant's Spoon" and you had a design of an elephant holding a spoon in its trunk. Using either the elephant or the spoon by itself wouldn't be a problem, using them together though, could very well be.
Use your best judgement in that situation, but if you've signed a rights transfer then consider it off limits unless you get permission first.
That's my two cents.