Yes, it is. Mellel is a very good and capable word processor, although it has a somewhat steep learning curve. It has lots of features that were very well implemented. The problem with Mellel, as I see it, is that it uses a proprietary file format and that it is developed and sold by a small company called RedleX.
If the company runs out of business for some reason, you will be stuck with your Mellel files with poor compatibility with Microsoft Office. You can convert a Mellel file to Word, but the conversion is poor and you will lose much of the formatting.
Plus, Mellel is only available for Mac (and iPad), which means you are pretty much confined to Apple's ecosystem. Sure, you may love Apple and the Mac environment. However, if one day you want or need to switch to Windows, then you must drop Mellel, and you will have to deal with all the compatibility issues with other filetypes.
I stick with Microsoft Word. It is a fantastic word processor with tons of features and keeps getting improved. Plus, it is the most used word processor in the world, which means I will never be trapped in incompatibilities.