Sure, they are most likely have some value, depending.
Whenever I take a class, I like to see the syllabus, get a review on the instructors and figure out what my goal it.
A "general" class on diesels will be general, and probably great for a beginner. Will most likely tell you how they work, basic maintenance, typical operation and issues.
A Yanmar class will teach you specifically their engines, but not all of them. However, you'll probably learn more.
A class on the CAT 3306, you'll most likely get down to the nitty gritty. You'll learn exact specifications, limits, parts etc. on THIS engine. Perhaps a lot more valuable.
What I often do, is to hire a mechanic that specifically knows MY engine. And, I'll have him teach me every operation that I'll expect to do on that engine. If I plan on rebuilding an engine, I want to rebuild one under supervision first.
For most of us, learning the basic maintenance is enough. How to change oil, filters, hoses, belts, zincs.... perhaps changing/maintaining a starter, generator, pulleys, turbos, heat exchangers. Perhaps more.