The six speed SA hybrid is reliable as any bike geartrain out thres. Yes, it has a derialleur, as do most bikes and it is as vulnerable to damage as any other bike out there. It is not really any bigger liability than just a plain Jane deraileur bike. My concern, and it is a small one is that you will end up with three shifters. That can gets a bit confuing, but that is not a deal killer as far as I'm concerned.

The other suggestion of having a double (or even triple) chaninring expands your gear range and gets you into the same three or even four shifter delemma. When I first built my hybrid SA bike I had it set up with dished SA cogs in the rear, the Sturmey-Archer AW three-speed and a double chainring in the front. I ended up going to a single chainring in the front because of having to deal with three shifters and trying to figure out the shifting pattern. But I live on Long Island, NY. I am not facing big hills so six speeds and two shifters worked out for me. You may find a pattern that works and you get used to. And you are dealing with more hills than me.

Here is my ultra reliable, and old time index shifting S-A hybrid six speed moved to a different frame than my original build in 1980.