bcoin is an awesome project, i follow the development on github, this will bring many new features to bitcoin. And it is user friendly.
insight is user friendly also, this is why it had been used by bitcore to make their block explorer.
bitcore is an implementation of bitcoin core in nodejs, they forked insight and adapted it to be supported by bitcore code. Yes bitcore can be used with altcoins, so does insight, but the insight version of bitcore is more a UI version, it is called insight UI for bitcore, so it is just an adaptation.
The most interesting to work with is bcoin as it is a true bitcoin node in a browser.
could it be userd in others altcoin?
With some knowledge and patience you can turn any project to an altcoin compliant project.
In the case of Iquidus, it is really easy as it is made to be supported by Altcoins, for insight, if you have any problem you can fork it from some projects who aleready adapted it, and see how they did in the commits history, then adapt it to your own.
Or, build your own, it is not that complicated, just time consuming.