The Google Mobile-Friendly Test is much better than Bing Webmaster Tools to ensure our clients' websites are mobile friendly responsive. Google's tool is created specifically for most of the online searches from the internet, and most modern devices, modern browsers, and …
I always make sure that we are actively using Bing and Google. There is a tradeoff in your time when you use more tools, but there's also benefit in multiple perspectives. And don't forget that Bing is a very viable search engine that receives 500 million search queries each month. You may find that your business benefits from a relatively small amount of attention paid to Bing because search ranking there is easier (not easy, easier).
I mean, it is a simple, no-frill test that gives your OK or NOT OK and highlights where it is. It is just a temperature check, this has to be used in conjunction with other tools for it to be useful. Since it is free I recommend running it as a check always.
The user-interface isn't as friendly or appealing as Google's - very bland overall.
The way they present their Page Traffic report is a big turn-off. It's basically just a grid of numbers and arrows which is hard to interpret at a high level.
Google's suggestions are sometimes vague or unhelpful.
The test doesn't offer multiple screen sizes or shapes. Though this isn't a huge issue, it would be nice to make sure a site using media queries stays mobile friendly throughout size changes
I think that Bing does a good job of being a competitive product in a space that has very basic and insanely complex products. While Bing isn't pretty, it gets the job done and also gets you the right tools in a simple and straight forward backend UI. The ability to place Bing ads, track SEO and see how your site is performing, all from one backend interface is awesome!
I think this is a great addition to other search tools and a really quick and easy way to check how your pages work on mobile to ensure your users have the best experience.