Lanefan
Victoria Rules
And right there you hit the first problem: that having designed that universal language you then have to shoehorn everything into using it even when it's not the best tool for the job. IMO that's bad design.All of that being given there are a few things that good, simple, elegant rules systems can do for you. The first is that they can provide a UNIVERSAL LANGUAGE.
The drawbacks you list don't outweigh the benefit of this sort of design always either having or being able to have the best system in place for whatever is needed at the time. Further, with less integration it's way easier to change or tweak or add or drop something such that the game works better for you.That is, in a game system like old school classic D&D, like say AD&D 1e, there is a big problem. Every single thing that may come up in the game is represented by some completely different subsystem, which uses different kinds of dice, etc. Nothing can be compared to or integrated with, anything else. There needs to be a rule for every specific thing, like "what happens to my initiative if I'm injured?", or even "what is the benefit to initiative of a high DEX?". This quickly increases complexity and creates both a burden on the GM and uncertainty in the mind of the player as to how the fictional world actually maps onto the resolution system (and thus uncertainty about what their abilities mean).
How boring.D20 was a huge advance in the sense that it unified all of these things. Now I simply know that everything dealing with how quickly and accurately I can move and act is a check modified by DEX, and it is all done on a d20 against a target value, and there are a pretty small number of standardized target values (in 3e it was AC, or a conventional set of difficulties, 4e got even more precise about this, and 5e is equally precise most of the time). If some completely novel situation comes up, the GM and player already have agreement on how it is handled, and the player can evaluate his options with some confidence that the GM won't suddenly require an unmodified d8 roll needing an 8 for success, or something like that.

The DM should always be able to rule that situation-x requires an 8 on d8 to succeed (if only because you just can't get an accurate 1/8 chance on a d20) - but she then has to be consistent and do the same again if situation-x ever re-occurs.
The risk there is medium- to long-term boredom once players (and DMs) figure they've learned all the tricks.Likewise the benefit in complexity budget should be clear. If a player need only learn one core system, then they are ready to play almost right off the bat. Their investment in mastering rules is much reduced, and you're vastly more likely to get people into the game than if it takes 100 hours just to fully understand how combat works. There are going to be a few people who find such simplicity offputting, but in this day and age when people have vast amounts of calls on their time, it is almost surely a winner.
Presenting lots of different elements isn't a problem in itself provided one can write both clearly and 'interestingly'. The key there is to have a useful index and coherent structure in both the player-side and DM-side rulebooks, which is where 1e-as-written falls down hard.Another advantage is in terms of presentation. While the details of presentation are closely related to the 'content' side of the equation, they are certainly made more tractable if you don't have to present a vast number of different elements simply to cover all the rules system bases (or else simply leave most of it to the GM to slog through). This is the nature of the vaunted GM friendliness of 4e. It is a real thing too, even its fairly simple stat blocks give you a monster that can interact with every rule in the system, as needed.