It think they'll be okay
Microsoft are going to make their phones compatible with all apps.. so I think they might be ok.
I despise the Windows phone interface, but only a complete idiot would think making a phone compatible with all apps from all operating systems is a bad thing.
Android apps will run without the user having to do a thing. Download the app, run it. That's how it will work on a Windows 10 phone.
Supposedly iOS apps with have to be recompiled in Visual Studio - you load your project into VS then compile it as a Windows phone app. That's it, no changes, no modifications. We'll wait to see if that works (I predict issues).
Most people producing iOS apps aren't doing so on PCs and therefore don't have Visual Studio. Problem? Not really, Microsoft are about to distribute Visual Studio for Mac OS and Linux at a cost of zero dollars and zero cents. Because they want iOS app developers to put their stuff on Windows phones. Don't hold your breath Microsoft, Apple users tend to have a slight bias against your software.
So potentially, if you can get past the awful UI, Microsoft phones will be the most flexible phones on the market.
Here's a fun fact. Connect a Bluetooth mouse and keyboard to a Windows 10 phone and it will ask you if you want to run Windows in desktop mode. Not sure why you'd want to run 6" Windows PC.. but you could.