Just finished a month of shooting with 5D3 & ML in Cape Verde for a feature, over 1TB of footage (H.264, not RAW), not a single crash or issue.
I absolutely love ML and the tools it gives to a DP is invaluable! Highly recommended, I could not go back to Canon firmware.
BTW, the sensor never heated over 53C (even at midday Cape Verdean sun), the active cooling just kicks in and drains your batteries a bit more. But I've shot in similar situations with Canon firmware and it's just the same.