Has anyone installed custom ROMs on mobile phones? On Windows I have used Odin and recently heard about Heimdall. Does anyone recommend it and if yes, is it suitable for the Software Boutique? I really need an alternative to Odin and this one is the only one I have found.
The Software Boutique should be a place for selected software that is proven to work with Ubuntu MATE. It should not be a replacement for Ubuntu Software.
My guess is that Heimdall is only for a selected few. So, IMHO it should not be added to the boutique. You can easily install it as it is in the main Ubuntu Repository, just do a
apt search heimdall-flashI’ve heard of Heimdall, but haven’t seriously used it. From what I remember, it’s visually similar.
When flashing phones I tend to “stay safe” with a tool I know definitely works – Odin – and that involves pulling out a virtual machine. 
I echo @wizd3m. I think this tool is quite specialized then something that works for the average user. Flashing phones isn’t something to be taken lightly. 
My tuppence worth on this one is that the phone makers should also provide software FOR ALL OPERATING SYSTEMS and not just Winblows/MAC, write them a strong letter demanding support for your chosen OS or you will complain to the monopolies commission of your country!. 