Nice, unified and symmetric. Except of course when it's not - only positive modifiers affect languages known, and reaction adjustments from Charisma only go from -2 to +2, and ditto for initiative adjustments from Dexterity. But we'll get back to that.
While this is probably the smoothest system in any official edition - more unified than Original and Advanced D&D's jumble of adjustments, and less game-deciding than WotC D&D's gigantic -5 to +5 range, they're not the best choice for everything.
Negative modifiers can be problematic when you want to modify numbers that are naturally quite small. Rulesets will often include things like “you can hold your breath for a number of minutes equal to 1+CON”, which will then require special stipulations and minimums for negative modifiers.
Also, I have witnessed applying modifiers to dice rolls cause headaches to OSR hackers creating their own games using B/X as a starting point (often via Lamentations of the Flame Princess). ‘Esoteric Enterprises’ and other games by Emmy Allen use X-in-6 skills like LotFP, except ability modifiers are applied to skills - though they can never go below 0-in-6 (rolled as a 1-in-36 chance). Since skills start at 1-in-6, this means that a modifier of -3 or -2 is the same as -1, not to mention that a +3 is pretty massive. The WIP post-apoc game Ruinations by Brent Ault has gone through several iterations with its skill system, likewise attempting to include ability modifiers into skills, but trying to dampen their effect. At one iteration of the ruleset, skills were moved up to the d12, starting at 2-in-12, plus ability mods. Once again, anything below a -1 is not accounted for. The skill system was changed in a later version to a d100 where start skills at 20%, and have each point of modifier count for 5%, so a -3 modifier would give a 5% success rate. Which, you'll notice, is mathematically the same as a d20 roll with the modifiers applying in their usual way.
Basically, the d6 skill system is liked (by me and many others) for its chunkiness - adding a pip to a d6 feels much better than adding a handful of points to a d20 - but big modifiers and big chunks don't mix.
I'm going to suggest something to all hackers, tinkerers and homebrewers right now:
Ditch the negative ability modifiers from your game. Completely.
It's okay. Just because you're using B/X as your engine doesn't mean you NEED to have the same ability modifiers. It doesn't break compatibility. You still have the same scores, in the same range of 3-18, for when you take ability damage or whatever. You can still run Keep on the Borderlands even if there isn't some unlucky geezer running around with a -2 DEX. You still have HP, and XP, and AC. You can still use all the great TSR and OSR content out there exactly the same.
Then, squish down the modifier ceiling to one that you think won't break your maths too much.
Yes, it may be somewhat more fun for players to have wide variation in characters' abilities, and amusing to laugh at the one chump with a big negative modifier. But reducing modifiers to a range of, say, +0 to +2, opens up a lot more design space for a homebrewer. Never again will you have to worry about special stipulations when applying modifiers to a base number of 1.
(Btw, this pairs quite well with the static Health/Wounds mechanic I talked about in an earlier post - I'll just have CON modifier increase your starting Health at level 1, but more on that in a later post.)
As for precedents, there are already places where B/X et al. restrict adjustments from abilities to -2 to +2, like reactions, initiative, and XP adjustments - because a +3 would be far too large a modifier on a 2d6 roll, for example. Why not expand these limits to everything, thus truly unifying ability adjustments? A smaller range of ability modifiers that excludes negatives means doing less maths, a larger design space, and fewer special cases.
I propose the following modifiers, and will use them in my next game:
Essentially, it's the B/X range but with negatives completely removed, and +3 squished into +2. You still have some characters (21.3%) who are very good at a given thing, and a few (4.6%) who are exceptional at it. The rest (74.1%) are just average. And that's okay. Now, your base Bushcraft (or whatever) chance will be either 1-in-6, 2-in-6, or 3-in-6. No special stipulations.
By the way: in the oft-referenced late Gygaxian houserules for OD&D, abilities modify things by +1 or not at all. And they modify very few select things. Constitution of 15 or more gives +1 HP per HD, and so on. In discussions of these rules it is often pointed out that they were made for convention games, and therefore do not represent how Gygax ran the game at home. It is true that many of the changes there improve PC survivability - which makes sense when running a quick convention game. However, the streamlining and restricting the effect of abilities does not necessarily improve PC capability - and in fact set a lower ceiling for it than those in B/X and AD&D. I'm not going to say "if it's good enough for Gygax...", I'm just including it for completeness and to show that it's okay to do things differently to B/X - as long as compatibility is preserved. Compatibility must always be preserved.