In terms of web engine, there are Gecko(Mozilla) and WebKit(Apple) for you to chose from, which isn’t that much choice.
I use Mozilla every single day and have done so for about 6 years now. Personally I don’t feel like I’m missing out on anything but ads if I don’t a Chromium based browser.
It is viable, just lack diversity in terms of web engine. Web tech today are just too complex to have another web engine. Even Microsoft ditch its EdgeHTML and favor Blink, the engine used by Chromium-based browsers.