Hi bashirs,
I look forward to hearing any updates. In the mean time I have done some further testing.
You have said that 'most visuals work properly in sandbox with no changes'. With this in mind, I have taken a copy of the Linear Gauge from the gallery and created a new version using Dev Tools. The only change I made was change the default color of the gauge to make sure I was looking at my new one. I then created a new report using the new custom visual and deployed it. The gauge displays ok on the web site (both report and dashboard), but it does not render on iOS mobile device. This is exactly the same behaviour I am experiencing with my modified visual. This proves that this visual does not work in sandbox with no changes.
Also, I have opened chrome developer tools while displaying a report with the custom visual, and there are some errors being thrown. Here is a screen shot...
Your help is greatly appreciated with this problem. We have just finished rolling out a solution to a global company, and they are rightly getting very annoyed, to the point that they are considering ditching Power BI. We have had many other issues as well as this (related to iOS bugs and data refresh issues).
Finally, can you tell me where on the https://app.powerbi.com/visuals/ web site it says that custom visuals are a preview feature, as I can't see that mentioned anywhere.