Doubtful on the second point (show me a modpack that is more stable the LoR -other then BUG which comes with an installer, I can't think of one, and neither can you). As to your overall point, there are exceptions to every rule, but really in general I stand by my assertion. I mean look around, there are literally 1000s of mods out there, most of which are pretty messed up. There are probably only a dozen or so with installers, and pretty much every one of them will work correctly and are well built.
So in general for your average user, it's just simpler and easier to stick to mods that have a straightforward install process, ie, download and install. If they do have an interest in a specific mod that doesn't have an installer, they should of course try it, but at the very least the modmaker should have detailed instructions on how to install the mod (where to put it, if it needs to be extracted, etc.), if they can't even do that, you can be pretty sure the mod is simply not worth checking out.