I don't find rests in general to be that big a deal.
You're an active participant in an excessively long thread about the issue. ;P
I think this is the easiest edition of D&D in regards to recovery, but I don't know if the way it's set up must be so restrictive or that it has to favor one class over another based on how they are designed.
The class designs are such that some will necessarily be favored by shorter days, yes. That's an inevitable consequence of designing them with different mixes of resources. Some classes have few resources, some many, some that recharge mainly on short rests, others long, others both - some classes resources are inflexible, others extremely versatile. That's a lot of complexity to balance, and the tight-rope the designers chose to walk is secured by the 6-8 encounter/2-3 short rest guideline.
As DMs, we deal with the consequences. Some of us are content to walk that tightrope, others put up a net, or just never get on it in the first place.
I find that most such complaints tend to be exaggerated, or that they can be resolved or at least minimized to some extent but for some reason such solutions are dismissed by those making the complaints.
Complaints & how they're perceived are, of course, subjective. There's a segment of the fan-base that couldn't care less about balance among classes or estimating encounter difficulty, so, of course, see no problem when deviating from a guideline that's meant to help with both. (There's another segment that cares, in the sense that they loathe class balance, of course - the guideline is thus a sort of compromise.)
Resolving and minimizing those issues is possible, but the point of this thread is that, while the guideline for doing it in a fairly straightforward manner is provided, it's not so well-supported by the mechanics (my point, above), system artifacts (spells like Rope Trick et al), or APs (Zapp's main point, IIRC).
It seems to me that the 3-18 encounter guidelines
It really is 6-8. I know you put a lot of stock in the fact that you could design a 3-encounter day with the same exp budget, but it won't have the same "balancing" impact on a class with some 3/day ability as a 6+ encounter day would. In theory, you just need to /average/ 6-8, which'd mean, sure, have the occasional 3-encounter day (or even 1-encounter day), but 'balance' them with equally-frequent 9 and 15 encounter days. Of course, if you're finding it problematic to pad a day out to 6 encounters, 9 or 15 or 18 is probably out of the question.
actually allow for more flexibility in world building than single encounter days would.
Neither sound terribly flexible.
Flexible would be 1-
n encounter/day.
