It depends on what you are trying to achieve. To some "gritty" just means slower / more difficult HP/HD recovery.
However, the rule as written also limits classes that rely on long rest recharge more than short rest recharge. Though that depends on the pace of play. I you can't recharge your spells for a whole week it definitely changes things - but is that "gritty realism," IDK?
FYI, you are not required to have 6-8 encounters per day to challenge your PCs in O5e or A5e. We regularly only have 1-2 encounters per day and I find it rather easy to challenge my group.
First, sorry, I meant to get back to this before now, but it was a busy week at work.
Second, let's not harp on the semantics of the phrase "gritty realism" or the individual words in the phrase. I'm using the term to mean nothing other than the rules changes for rests and recharges above, and nothing else. And I'm using the phrase because that's the phrase used in the original 5e rules (in just that one location, as far as I can tell.)
Finally, what am I trying to accomplish Fair question. I'm trying to change the pacing.
Basically, the mechanics of a rest economy imply (and to some degree enforce) a pacing of encounters that I don't like, and I'm trying to change it. Since 4e introduced the idea of daily powers-- and by extension, the idea of the rest economy-- the basic idea of encounter pacing has been "Your group should be expected to get through X encounters per day, broken into smaller groups of Y encounters, punctuated by long and short rests, respectively, to recharge abilities."
It's a decent enough model, except it normalizes the idea of X encounters per day, because that's how many encounters it takes to burn through the party resources to make things feel challenging.
Your pacing mileage may vary, but for me that seems weird. If I send the characters on a quest to find a lost city 150 miles into the trackless desert, it's going to take about a week to get there. I might not need to have an encounter every day, but if I do, I either need to have X of them, or I need to make the one encounter apocalyptically challenging. ("But don't the characters have to assume that the first encounter of every day is encounter 1 of X and conserve resources accordingly, ratchetting up the challenge on their own?" Good question-- Yes, but that only lasts until the players figure out that I don't want to have that many encounters per day during travel time.)
On the other hand, if I'm using Gritty Realism, that week of travel will support the same number of encounters as a day. This absolutely does change the artificial one day quantum of adventure time to seven days, but (1) it still feels more natural to me and, (2) my style gives me more tools to adjust the pacing of overland journeys than dungeon crawls. Tessarael raises the valid question of overland travel vs dungeon crawls. To that, I can only say: (3) I tend not to run big dungeon adventures. Never found them realistic.
If I were just interested in running an original 5e game (or probably a Pathfinder 2e game) I wouldn't sweat it too much.
But the fatigue/strife/supply rules are a set of wrinkles I'm not sure how to account for.
As for the difference between long rest/short rest powers, does this
really disadvantage one more than the other? The classes limited by long rests are still limited by long rests, they are just farther apart. But so are the short rests.