"It's good until it's not" expression

I came across the construction "X to be Y until it [not to be Y]" several times..For example in Silicon Valley TV Show:

That was pretty badass Richard, until it wasn’t

and I always thought that it literally means "X to be Y until something happens". Recently I tried to use this construction in the article named "ES6 is great, until it’s not"(ES6 is programming language) and the main point of the article is ES6 is OK to use as soon as it does not bring serious problems. However, I can see from the comments and replies that many perceive the content like I am trying to discredit ES6 which was not my intention at all..I suspect that it happened because the name does not actually means what I thought it means..So, I need help - what's the actual meaning of "X to be Y until it [not to be Y]" and whether it has fully negative connotations?

There is a strongly negative connotation to 'until it's not'. It implies that a person will be blindsided by a (often sharp or drastic) turn for the worse or that they will become disillusioned by the reality of a thing. The part that comes before, in your case "is great", doesn't counter balance the 'until it's not', in fact it has a tone of sarcasm, it's commenting on how the 'great' doesn't mean much after it takes a turn for the worse.