Both words are adjectives:Sincere
- To be an outspoken critic /opponent of something
>> Criticar algo /opor-se a algo abertamente.
Although the first words are considered synonyms, I believe there is a slightly difference and we realized it especially when we need to talk about someone, or even something which is causing trouble for us.
Sincerity is the contextualization of a problem caused by another, but the complain about the person is more polite than a simple outspoken critic. When we are outspoken of someone, we were invited to present a direct exposure of the problem, regardless of the consequences. To be outspoken is to tell someone point-blank what you consider negative.
When someone is sincere, the main worry is to save the one who is called from personal judgment, persuading him to avoid such negative action. Someone can be spontaneous and guided by strong emotions when is outspoken of someone or something. Sincerity is worked out to be adapted to every circumstance.
Imagine a situation where you need to tell someone about his / her bad breath. Thanks God we, computer users, don't need to worry about that! I'm joking
REFERENCE: Pearson Education. Longman Dictionary. 2004.