You can use Hybrid Assistant, while driving, to check that the engine temperature stays within operating limits.
While this is usually not an issue in a urban environment, driving on the highway is not always considered safe.
Using Hybrid Reporter, you can check the engine behavior: here is a test of 20 kms at 130 km/h indicated speed.
The engine coolant temperature settles on a safe value of 90°C.
We can see that the electric engine temperature was raising, but its operating temperature is way higher that the petrol one, beeing able to work until about 160°C.
A higher trip may show if the trend will slow down and settle on a stable temperature.