The US far left is not scary at all. It barely exists, and it has very little traction among those who feel they have little to lose. A lone pissed-off individual might try to shoot some cops or a congressman, but there's nothing like the organised, if fractious, effort you see from the far-right.
The way I see it, in the foreseeable future the danger is of individual Oklahoma-bombing type terrorist acts, and lots of much smaller acts of violence against specific minorities. Overwhelmingly from the far-right.
I have no clue about the long-term. I don't think any country can claim it's necessarily immune, forever, from a descent into Syria-style chaos. But there would be a very long run-up to that situation, certainly involving some huge economic shock or two - it's not going to be triggered just by Trump being impeached or losing an election.
But I would say the existence of Trump, and the intensity of his support, is a pointer to how Americans can turn irrationally angry and ultra-polarised just like everyone else. I take it as a warning sign of where things could be going in the very long run. I still think economics is primary when it comes to large-scale disorder.