There aren't really any boundaries.
You have: American hardcore Powerviolence Crust Euro hardcore
And about 100 other subgenres, that have their own styles. It's mostly just energy speed and pissed off.
For example bands like Brain Tourniquet, storm of sedition, fentanyl, candy apple, tower 7, kinetic orbital strike, kohti tuhoa, extortion, Woodstock '99, Doped Up, and my current obsession 偏執症者 (Paranoid) all fall under the Hardcore label but all sound distinct from each other.
Edited to add links to band pages