Is there a hardware or software limitation to how long smartphone can record video? Besides the obvious battery and storage limitations, are there any limits to how long can a smartphone record video? For example, could the image sensor overheat after an hour or more? Would the camera software explode?
In theory no limit on recording. But as you say overheading is a big problem on summar days.

I experience that camera did shutted down recording video because a software problem. When you recorded, everyframe have to processed by processor. And camera recording process is a stream between processor and camera chip. During this stream, if a bit of stream's data changes, processor will handle an error in stream and will delete all recorded stream. This is a one scenario. Scenario's count can be increased. 

For example if processor heat up, and after heating up , on transistor level will one electricity jump 1001st transistor to 1002th transistor and this cause a processor error and this can be the limit of recording video. 

I hope I could explain.