Many embedded systems tends to interact with the outside world. When this happens the embedded systems engineer will acquire knowledge about that specific field and that may not be usable outside it.

I, for example, have lots of experience in embedded systems. Specifically with electricity metering. Of course I can design other kinds of embedded systems. But through the years I have acquired knowledge on metering algorithms, sensors specific for metering and the industrie's protocols and terminology. If I suddenly move myself to the automotive industry many of this knowledge may be useless.

But there's a good side to this. When you have this specific industry knowledge you are more valuable to the companies on that field. Since it is not easy to find someone with that skills on the market.

That's also the reason why you have to be very good with the basics and fundamentals. Those will not vary from different fields of application.