I'm Jean Labrosse, I wrote the uC/OS series of kernels (uC/OS in 1992, uC/OS-II in 1998 and uC/OS-III in 2008).
There are quite a lot of things to like and in fact prefer over Nucleus. For one thing, I wrote a book that accompanies each one of the versions (I'd recommend you look at uC/OS-III). We have 7 variations of the book:
uC/OS-III for the RX62N, STM32, Kinetis, SH7216, Stellaris, NXP and Infineon. Each book is about 950 pages and is available either for FREE in PDF format or available as a hard cover book:
Each book explains the internals of uC/OS-III and provides examples using one of the above 7 MCUs.
You can also download the full source for uC/OS-III from our website so you can see for yourself the level of quality that went into the code.
If you need more than just a kernel, you will be happy to hear that we also have a full suite of protocols tasks: TCP/IP, USB-Host, USB-Device, Modbus, CANbus, File System and GUI so, the kernel is just one part of our offering.
In fact, similar to the uC/OS-III books, we also have 5 flavors of our uC/TCP-IP books:
We support just about every MCU and CPU architectures available on the market.
BTW, in early 2015, we have been named the MOST popular RTOS on the planet by UBM (United Business Media) ... assuming you combine uC/OS-II and uC/OS-III). In fact, we were some 10x more popular than Nucleus!
I urge you to check out our website (www.micrium.com) as well as a tool that can be incredibly useful called uC/Probe which works with or without an RTOS and in fact, any RTOS even non-Micrium.