If you're doing any significant touring on a splitboard, dedicated backcountry boots are worth it. The big difference is the walk mode—a mechanism that frees the ankle for natural uphill strides, then locks down for the descent. Regular boots force you to hike in a fixed, flexed position, which gets exhausting fast.
Backcountry boots also feature lugged outsoles with deeper tread for grip on rocky or icy terrain, and they're built lighter to save energy on long approaches. The stiff flex ensures you still get responsive control on the way down, even in variable snow conditions.
If you're only doing short sidecountry laps or occasional tours, a stiff freeride boot can work in a pinch. But for full days in the backcountry, the touring efficiency and comfort of a purpose-built splitboard boot makes a real difference.