And at the same time, everything gets a whole lot blander.
An example, perhaps:
There's lots of things in the game - spells, feats, magic items, etc. - that affect how a character can or does move: haste, slow, jump, climb, flight are just a few.
The unified-design theory would probably want a one-size-fits-all rule, something like "non-natural* movement effects of any kind or of different kinds cannot stack", i.e. that a creature or person can only have one such effect active on itself at a time. Easy, sure, but boring as hell and, IMO, very lazy design. (a second, corollary, blanket rule would be either A: that having one such effect active denies the application of any others until the first effect has ended, or B: that application of a second such effect overwrites and ends the first)
Far better would be to delineate how each of these different "extra" movement types can or cannot interact with the others, and also with additional instances of itself. Can someone be double-hasted or double-slowed? Do haste and slow on the same person just cancel each other out? If two different things give a character the ability to jump 20 feet straight up, do the two together allow a 40-foot jump? Can a hasted character be given spider climb as well? Etc., etc.
And then start looking at specific feats or items or spells and see if any exceptions then apply to the answers to all the interaction questions above. For example, does wearing boots of speed function differently than a speed-you-up spell? Do slowing effects stack while hasting ones do not? Etc., etc.
And if this means some write-ups get long, so what?
* - the "non-natural" is there to allow for built-in natural movement abilities e.g. an Aarakocra's ability to fly.