Actually, I'm not even sure that it puts all armies on a level playing field any more than they were before allies.
If before, we could divide armies into two levels, "from a good codex" and "from a bad codex" (an oversimplification, but useful for the point),
now we can divide things into THREE levels:
"Armies from a bad codex",
"Armies from a good codex or from a bad codex that shore up their weaknesses with allies",
and the newcomer,
"Armies that combine units from different codexes to form something really broken". Like the various "stars".
Now, your fluffy pure-ork list might usually lose against an army from a good codex (or an army using some allies), just as it would have before. But if you practice, you might win sometimes, and you will usually have at least a fun game.
But if you take your fluffy pure-ork list against like a tuned Jetseer council or something, you're just wasting your time. The outcome is never in doubt at all, and you might not even kill anything.
The increased tools available to everyone have ALSO increased the ceiling of how powerful and broken an army can be, so "normal armies" find themselves just as far behind as they were, and "fluffy armies" are almost playing an entirely different game now.