NeXT used Objective-C + events. The concurrency model is the major unforgiving factor, though apparently the NeXT Objective-C language and tools were ahead of the time in many ways as well.

My project supervisor had a Cube on his office, and my thesis was to port a particle simulation framework from Objective-C into Visual C++/Windows 98.

Using Objective-C is just like C++ in regards to shooting yourself with the C subset common to both languages.

Foundation did not have that much support for heavy multi-processing. At least not at the same level as OS X.

It was all about UNIX programming model + Mach ports and tasks.

If you want to read about it, this is one of the few places where the documentation is still available online.

http://www.nextop.de/NeXTstep_3.3_Developer_Documentation/

Interesting!

I grant that NeXT would have been more difficult for develop for when you did require parallelism for running compute intensive tasks on multiprocessor systems, but this was not the NeXT target market. BeOS got the ability to do parallelism by choosing the more fragile concurrency model, and apps paid for it in all cases. NeXT didn't do parallelism in the GUI apps, and was easier to develop for as a result.