This is the thing I'm looking to address. If nearly everything was short rest, or if everything was long rest, then you'd only have to stress about balancing the encounter or the day. Balancing the encounter is easier, in my opinion, because balancing the day requires restrictions on when people can rest. If you do a 4E style game, where a short rest is measured in minutes instead of a full hour, and you expect a short rest after almost every fight (except gauntlet/reinforcement fights), then balancing the feel for easy, medium, hard, and deadly becomes much, much easier.
Take Red Hand of Doom for example. In 3E, a day was balanced around 4 moderate encounters in a day. One of the set pieces is taking out a keep that the bad guys have holed up in. There were 4 encounters set up in the keep, and one couldn't reasonably start attacking the keep then stop to take a rest.
For 5E, players can only really handle 2 moderate encounters per short rest, and need 6 moderate encounters to hit their daily xp threshold; going under that will make an easy day, which then makes it difficult to make the adventure feel perilous. 5E PCs can handle maybe 3 deadly fights in a day, with a short rest between each, but with those taking a full hour, you have to stretch things out.
In 4E, players could handle 4 moderate encounters in a day, with a short rest after each one. That same keep is very easy to convert to 4E.
I can just convert the keep fight as is, and just accept that it will be an overly difficult fight for the short rest resource characters in the party, but balancing all classes around the same point would be far, far easier.