Well, I agree that just tossing advantage/disadvantage out there as some sort of blanket thing, or something you can just 'turn on' etc. is bad. At least in the design of my game it isn't THAT easy to get! I mean, if you want to arrange your tactics properly, which may involve some difficulty or risk at times, then you CAN probably get advantage OFTEN at the time and place you really want it most.Physical environment can play a role here too - you could be fighting on ground consecrated to an opposing deity, for example, giving +1 on everything (to-hit, damage, saves, checks, etc.) to the ground's defenders and -1 on everything to you, the invaders; never mind that your divine spells or effects are greatly reduced in effect here. Somehow draw the defenders off of their sacred ground and fight there, and these factors go away.
Simply saying all the defenders have advantage and all the attackers have disadvantage while on that ground doesn't work for two opposing reasons: one, if advantage is easily cancelled out by disadv (and vice-versa) there's so many ways of doing so that the whole thing becomes almost moot; or two, if adv and disadv cannot easily be cancelled out then it's way too big a benefit/penalty.
I still say other things are probably better choices and more interesting. As HoML is written, if you granted, say, a +1 permanent bonus as an environmental effect, it would still stack/not stack with other sources of said type of bonus (though it would probably also be a bonus to X, where X might be attack checks, defense checks, or perhaps some other sort of checks). So, some characters might NOT be measurably better off, but it won't hurt them and if its sufficiently beneficial for the party overall I don't think that sort of thing would be a big issue. However, I could just as easily see an environmental effect being something like "Free Action, once per turn, remove the dazed condition from your character." or something like that (obviously you'd want to make it a condition that the scenario probably inflicts fairly often).
I'm not categorically opposed to the use of bonuses (or penalties perhaps) as a tool. I just think its always worth considering first if something else is going to serve equally well or better.