Hi!
Well, my concept is a little different indeed.
Music does not work well with complete randomness and interactivity.
For “full” flexibility, then the everloop asset works best since you can easily just drop another custom sample into the mix as long as it’s composed in a similar manner to the other ones and in the same key, it will blend in easily.
My asset is a lot more complex. The idea was to create music which sounds like a regular pre-composed soundtrack but which is in reality reassembled using a random set of samples. On top of that, you have a certain level of interactivity depending on the music style.
My code is not very flexible because I customize the code for every single music style! 
As you have rightfully noted, in the “electro” asset, the samples simply play in a loop and are faded in and out (the first video that I attached above). In the “medieval music” from which your screenshot comes, the song structures are precomposed in code (for instance, on measure one, play the flute and drums, on measure 5 play the drums and horns) so that it sounds like an actual live band. There are several song structures which are chosen at random as well so you never have the same song structure and the same combination of samples twice. The transitions are not fade in/fade out, except when you ask for the music to stop, because it is meant to emulate a “real” live band, so the music stops at a natural “breaking point” (the end of a melody/loop usually). 
In terms of flexibility, I did not provide any type of UI to drag/drop custom samples.
The code fetches the samples directly in the folders and assigns them to arrays.
If you replace all samples with your custom samples, it will probably sound weird because the samples might not have the same length or will be played within a song structure which will sound strange.
For the flexibility you require, the best solution is probably FMOD, Elias or Psai. The only thing is, I’m one of the only developers on Unity which does random/interactive music using Unity’s native audio engine, which means that you get the source code which you can modify if you choose.
I would also point out that since I use Unity’s audio mixer, if you prefer transitions which are based on fade in/out rather than the preprogrammed transitions, it’s easy to do using the audio mixer. Simply fade out the audio mixer’s “master” volume and stop the music once the volume has reached -80.0f. 
If you look at all the other videos of my assets, you will see that I developed custom solutions for each of them depending on the music style. For Rock music, the samples include precomposed loops with all the layers (bass, drums, guitar) and are played at random, for the Angels and Demons pack, the solution is simply to cross-fade between precomposed songs to modify the intensity, for the Endless Battle March asset, all layers are exported seperatly and recomposed in code so you get waaaaay more randomness… Each music style has it’s own limits in terms of what you can or cannot do to ensure that it sounds good, so I tailored my script to fit each… 
