CapnZapp
Legend
Please note we seem to be talking about two (slightly but crucially) different things.I've heard stories about banshees (CR 4) nearly pulling a TPK on a level 8 party because your non-proficient saves stay pretty much in the same place as they were at 1st level. And certainly Sly Flourish's essay about playing from 1-20 ( http://slyflourish.com/running_dnd_5e_from_1_to_20.html ) suggests that it's possible to maintain a consistent challenge across levels, but that the wonkiness of encounter building make it harder to do so over time.
Alas, I haven't had the opportunity to run a game beyond level 6 yet due to real life interfering, but I have noticed a trend in the low-level games I've run, which is that because combat moves so much faster than I'm used to from previous editions, I feel like the party isn't being challenged– until I'm reminded that at least two members of the group were dropped to 0 hp and one of them had two failed death saves.
So beware of falling into the idea that just because it was resolved quickly, that the fight was trivial.
5E combats are definitely very swingy tho. A lot of the fights I've seen were determined by which side had the drop on the other, even in cases where the victors, on paper, were horribly overpowered. Again, this can feel like the encounter was "too easy," when dice falling the other way could have made it a crushing defeat.
-The Gneech![]()
You talk about 5th Ed in general.
I talk about published modules specifically.
You, or I, or Angry DM are probably and hopefully all able to create challenging encounters for parties of all levels. As you correctly say, the best strategy is not to try to ensure that one particular encounter will be challenging, but instead to build several encounters, and then let the laws of averages ensure that at least one of them will (by pure happenstance) be challenging!

The sad part is that we would by this stage all be better than the ones writing the latter half of Out of the Abyss...!