No. PEX cannot handle boiling water for sustained periods. It will expand, kink, and potentially burst.
If you pour boiling water down your kitchen sink, it usually travels through the pipe and out to the sewer. However, if you have a clog downstream , the boiling water will back up.
Septic tanks rely on good bacteria to break down solid waste. Pouring gallons of boiling water down the drain kills these beneficial bacteria. If you kill the bacteria in your septic tank, the tank stops digesting waste. This leads to: