Back in the older editions, surprising a party with a bunch of a ghouls or a carrion crawler or a pack of wights or a gorgon or even a rust monster was bad news. Nothing's changed.
I think 5 or 6 years of 4E might have dulled some DMing instincts, at least mine. 4E was, for all practical purposes, a joke compared to earlier versions. Things were either deadly because the DM was throwing in encounters 4 levels higher, or because players made really bad mistakes.
We (or at least I) tend to forget the days when a group of PCs were exploring a Barrow Mound and they just knew that there were level draining undead in it, dreading to open a door. It used to be tense at times.
But even in 2E, the worst class to best class save deltas at level 17 was 7 for PPD, 3 for RSW, 4 for PP, 6 for BW and 3 for spells at level 17 with typical stat modifier deltas of 0 to 3 which did not apply to all saves. The mean save deltas (i.e average difference between any two classes) were 5 for PPD, 1 for RSW, 1 for PP, 3 for BW and 1 for spells at level 17
Everyone had similar saves with a few outliers.
5E has a 6 for the save delta and -1 to 5 for the stat modifier deltas.
2E, the typical delta range between best save and worst save is 1 to 6 (with a few outliers).
5E, the typical delta range between best save and worst save is 6 to 8 (with a few outliers).
There are no 5E classes with 1 to 3 difference for the base delta like there are in 2E.
So yeah, mathematically, this is a lot harsher of a difference. So, it might take some DMs by surprise. Granted, 3E save deltas were a lot closer to 5E deltas, but I do remember a lot of talk about that back in the day as well.
With bounded accuracy, I thought the concept was to flatten this stuff out some, but the weak saves are a case where the difference between best and worse on a D20 is really substantial, not much different than in 3E or 4E. The difference is that it is 4 out of 6 saves now, not 1 out of 3 (although some 3E classes had 2 out of 3 weak saves, but they made up for it somewhat with personal buff spells or multiclassing).