The chain path of an IGH or a fixie/singlespeed bike is really quite simple, and in the end, whether to use a masterlink or a one-piece chain really comes down to personal preference. Which is most convenient depends on your skills.
I've found that it's best to use a chain with a masterlink, simply because of the ease of removing the chain for cleaning. Using a chain tool takes me a bit longer than unclipping a masterlink. If you slip up and push the pin out of the chain, it can be very difficult if not impossible to replace it back into the link. (If you can even find the missing pin!)
I assume when you're talking about dialing in slack, you mean moving the rear wheel forward in the dropout?
The only time I had a masterlink break was, ironically enough, when a mechanic did the work for me. (Perhaps that was due to a bad masterlink. That mechanic in question was otherwise very, very good at his job.) Since then, I've learned that simply double-checking the link by putting pressure on the link is sufficient to test it.
With a bike where it's difficult to readjust the rear wheel (and therefore the tension on the shifting chain into the rear hub), I might say it makes more sense to use a masterlink. However, releasing (or installing) a masterlink with the chain under tension is quite difficult without a lot of practice. Practiced wrenchers can do this, but I don't quite have the knack. With an easier-to-adjust hub (like the Nexus 3), I might worry about this less and use a one-piece chain if I didn't have a spare masterlink handy when swapping out a chain.