I'm curious what the trade off is between a hard realtime kernel like LK and a kernel that only offers soft realtime guarantees. I would presume there are some

I'm curious what the trade off is between a hard realtime kernel like LK and a kernel that only offers soft realtime guarantees. I would presume there are some costs associated with providing hard real time guarantees ... otherwise every OS would be a real-time OS right? And if there are tradeoffs, why did Google choose a hard realtime kernel to be the basis of its new OS? Since Linux and BSD seem to be fine bases for mobile OS (as evidenced by iOS/Android).