My thoughts/opinion on Horde Nights 'scary' level.
Feeling "Scared" implies there's a threat you fear. More 'danger' usually = more fear/scary. Less 'control', more unknown = more fear.
So certainly familiarity breeds 'contemp', or less fear (old saying, not saying 7dtd is in any way contempable).
For me A16 allowed much more granular danger management.
How much you'd upgrade the blocks the zeds were trying to get through, what kind of slow-downs to use, granular control over various traps. Whether you just had a Claw Hammer or had a Nailgun influenced choices to a meaningful degree.
There was a 'pace' to the zeds damage that allowed just enough time to recognize that you were about to lose a wall/defensive position, and react to it.
My best guess, and personal opinion, is that the zombie group block damage change is the major cause of the dramatic shift in base defense between A16 & A17.
First few Horde Nights in A16 you could both put out enough melee damage, and repair enough, that compared to the zombie damage output you were still in the race. Not so much in A17, and certainly not, imo, later on.
This led to extremes. Either your base would survive in A17 or it wouldn't. Rather than large bases for later stages in A16, it seems like A17 really encourages smaller bases. And lots of traps to overcome the significant damage output the zeds have over the player (SP here).
And while A16's zed damage & AI allowed the player to somewhat manage the damage their base took; leading zeds to a different face and being able to repair blocks during lulls, A17s version of that is much smaller/harder.
I don't have any great suggestions to offer. For me, though A16s HNs were quite a bit more fun, it was certainly possible to make impregnegatable bases, and maybe that was the problem.
I know it's not fair to compare A16 with A17; they're so different after all. And I'm 'spoiled' by memories of A16s. A Spiked Club & a Nailgun accounted for >90% of HN usage (until the body cleanup, heh). In A17 I don't even have a melee weapon on my toolbelt during horde nights.
Just my humble opinion, but I think the missing secret sauce is more granular control over the chaos of HNs. Put another way, the difference between surviving and failure is too small. And it's resulted in folks building very over powered bases, to insure survival.
I am hopeful for A18. That MadMole is getting first hand experiance with later HNs can only be a good thing. And I'm guessing he'll get some quality hands-on with Traps on HNs.
Though I'm certain I've added a few pavers on my road (to hell, heh), I hope this is taken as itended; one peeps opinion and hopefully useful thoughts on what is a very important, but highly difficult to balance part of a great game.
Peace