Twitter is an example of a company that has had poor performance and may have been to immature to go public.
What exactly do you expect of companies that go public? What you wrote, and seemingly reflective of a common sentiment, is an extremely loaded statement of what anyone should expect of "glorious publicly traded unicorns".
Unless Twitter committed fraud by not disclosing something, then there should no really no other expectation or aberration in their publicly traded status. The banks, board and the CFO priced the shares and they've been printing money for all their employees using the deep liquid market that all these people fawning over tech companies perpetuate.
So I'm a little confused at what is so interesting about their poor performance. Have you seen the stock market, some companies' shares go up, others go down.
That said, a public company that has an incomplete product roadmap-- or poduct, shows no profit and has a poorly understood revenue strategy is immature. GM has been public for a comparable amount of time. It is well understood what they do-- they make and sell cars. Twitter aggregates and dynamically prices advertising and monetizes information. It is much harder to understand a new business model and one that is not making money. It was not ready to IPO.
Yes, the market goes up and down. Twitter was a shitty company, still is a shitty company and will be a shitty acquisition. I understood that and I didn't buy it. Thats how the market works.
Edit: While the market goes up and down, Twitter has pretty much just gone down. It was in the 50s 2 years ago and has pretty much just fallen.