Long days tend to make the game easier just like higher exp multipliers tend to make the game harder.
Based on time to loot useful items / Experience gained for the same time.
I half agree/disagree with this. I think the balancing on most sliders is pretty genius in that they neither make the game strictly harder nor strictly easier they just change the game in some pretty radical ways.
Consider, you said longer days make the game easier because you have more time to loot useful items and presumably also to farm extra resources as well as travel time to POIs or different traders taking up a lower percentage of time relative to shorter days. But consider also that longer days mean more real time until the trader and vending machines restock.
This means that early game with 2 hour days you'd be getting (relatively speaking) half the food and drink from a vending machine that a 1 hour day player is getting, making it much harder to survive off of those. On a longer day you're spending more calories per in game time unit, but able to buy less meaning you'll need more alternative sources of food which can be difficult in the early game before meat starts getting delivered to you from the friendly woodland bears and wolves. This also means you're getting half the resources a normal player is getting from traders in the same real time period. This means that things like explosives purchased from the trader become more rare as you're waiting twice as long for them to be restocked. Consider that while you may be doing 6 quests a day vs 3, I can safely finish those 3 with grenades and thus have more time outside of my quests for other things, where as the trader won't have enough explosives to consistently do 6 quests a day. This is also hugely impactful in the late game having to wait 6 real life hours to do a trader reset loop when checking for new books and such for sale from all your traders vs just 3 hours on default days.
Longer days also mean longer nights. This means potentially much harder horde nights. A 2 hour day means a 30 minute potential horde night if you play long enough, where as 1 hour days cap the night at just 15 minutes. Whatever strategy you're doing for horde night gets much more expensive and risky by doubling how long it lasts. Doing a field strategy? You just need 2 steroids max to off set an unlucky broken leg for the night where as you'd need twice that without. Using traps for the night? You'll need twice as many and need to spend twice as much time repairing them the next day. Etc. And for new players, the longer nights are also more dangerous outside of horde night in that they are likely not comfortable moving around at night thus stuck in place for longer, possibly with running zombies trying to break in and there is only so much organizing that you can do at night before you have a safe mine made to keep you busy all night.
For XP though, I mostly agree. More XP tends to make the game harder faster than you progress with gear and base (at least for the early and mid game). There are some very specific min/max strategies that you can pull off to optimize the fun out of the game and turn increased XP into an upside, but outside of those, the zombies tend to get stronger than you do if you are spending your points in a less calculated manner. Consider: it takes 9 perk points to go from str7 pummel pete 4 to str10 pummel pete 5. This represents a measly 7% damage increase for the club compared to increasing your base game stage by nearly 11. 7% damage is nice sure, but feral zombies have double the hp and damage and are now spawning much more often as your GS goes up. A player with lower XP multiplier will eventually get to the same perk level and GS, but also have way more time and thus more likely to have a much better quality 5/6 steel club which is way better than just 7% more damage on a wooden club or bat.
For optimizing the fun out of the game with extra XP, max daring adventurer on day 2 with just 150% multiplier (1 hour days) you can do this before finishing your 7th quest meaning you get a bicycle + 1 other bonus for the tier completion. You can then get better barter up to level 4 before the end of day 2 and thus have a huge chance of buying a tier 3 gun of your choice from your two traders that you'll have on day 3 or 4 for the reset. I did 6 test runs with a daring rush and had an auto shotgun on day 4 in all 6 no sweat. This means you already have the best gear in the game before the first horde night, so perk points just make that gear even better and the extra XP basically makes your daring adventurer and better barter free skills to speed up the game massively but in your favor. Combo this with points spent in miner69r to let you bust in POIs and finish all fetch quests that a trader has as well as easily finishing buried supplies (plus 1 point spent in the treasure skill in perception to speed it up even more) and you just snowball quests out of control faster than the zombies can keep up with. On all 6 games I was able to finish tier 5 completion and get 500 steel blocks or quality 6 and 5 military armor consistently before day 10.
Was it fun just busting into POIs and grabbing supplies for tier 1, 2, and 3 quests like 6-10 times a day and ignoring all the zombies and loot just to accelerate to end game gear against early game zombies? For me, not really. It was a fun test at first but it got old fast. The extra XP was instrumental in making the build as broken as it can be though, because INT is balanced by perk points actually having a meaningful opportunity cost, extra XP removes that and let's you get the two really strong skills (and spend all those points on the INT attribute itself) without giving up power in other trees thus allowing you to use the late game items you find more effectively.