Here are two examples from last night's session. (Keep in mind that 2 players were absent, so we had a more manageable 5 players instead of 7.)
Encounter Example 1: Bandit Attack
Party comes upon a group of 13 CR 1/2 bandits, 1 CR 8 Fighter/Rogue, 1 CR 8 Cleric, and 1 CR 8 Wizard. (According to the Encounter Calculator, that's a Deadly encounter at 23 Challenge Rating.) The party was moving silently and taking cover, approaching the bandit camp. On one side of the road were 7 bandits and the Fighter/Rogue captain; the other side was the wizard, cleric, and 6 additional bandits.
The party was sneaking and beat the scouts' Perception. They also beat most everyone on Initiative. Party's sorcerer dropped a fireball on the side of the road containing the 6 bandits and the cleric and wizard. He is out of reach for Counterspell. Bandits killed instantly. Wizard gets pincushioned by ranger's longbow. Down to 4 HP, wizard casts invisibility and moves to behind cleric. Cleric casts 5th level cure wounds on the invisible wizard. Some minor attacks are done by the rest of the party - barbarian moves up and rages and fights bandit captain and is surrounded by seven other bandits. Not a lot of damage is done to the barbarian.
So everyone made high stealth checks? How did they know the bandits were there and why did the bandits not have multiple guards posted? If this is a camp, why wouldn't the bandits have some kind of traps set up, even just the typical "bell on a string"? How does a group of 7 sneak up if it's completely barren desert? What were they hiding behind? Even at night, unless it's cloudy, they should have been spotted.
I agree that it's a deadly encounter assuming it's a normal encounter. But you stacked
everything in the PC's favor.
Sorcerer then casts Reverse Gravity from the wand - with enough range to get all the enemies. There is nothing but sand for the enemies to grab onto. The barbarian also flies up with the rest of the enemies. Sorcerer drops the spell. Everyone plummets 100 feet - but the sorcerer casts Feather Fall onto the barbarian, who safely floats to the ground. Everyone but the bandit captain is dead - and he surrenders.
Nuke the Reverse Gravity wand from orbit, it's the only way to be sure. This is an insanely overpowered item, far more powerful than any official wand in the book. There are a couple of staffs that cast 7th level spells but they're legendary items (although admittedly they do other cool stuff too).
Second,
this is a picture of the desert just outside of the Phoenix area. People greatly underestimate how much grows in most deserts, the Sahara is a very extreme example. Sure there are other deserts that are as desolate as the Sahara, but most of the Southwest US looks much like this. Not only would people be able to grab onto things (even if you're really going to regret grabbing onto that Cholla) there's also potentially issues with line of sight.
An example from a picture I took a week or so ago:
My take: Yes, the Reverse Gravity Wand is overpowered for their level, but honestly - what would've changed without it? Every bandit was already dead. The wizard and cleric were down to about 1/3rd hp after one round. At best, the fight would've gone one more round, taxed nearly no resources, and done a middling of damage to the party. A 23 Challenge Rating fight?
By my calculation (I use an alternate calculator, see the attached) it comes in at 129% where 100% is deadly. So only slightly lower, and given the OP nature of the party I'd calculate the PCs at level 8 or higher. The problem with the calculation you're using is likely that it's counting the bandits as multipliers. The bandits are such a low level I'm not even sure I'd consider them in the calculation.
But again, everyone getting a surprise round attack with a broken wand while having unlimited line of sight and I'm not surprised at the outcome.
Encounter Example 2: Lycanthrope Pack
Party is approached by a pack of 9 various lycanthropes (CR 16). As they approach, fireball dropped, killing half of them. Then - AND THIS IS MY MISTAKE - twin spell fireball a second time to kill off the stragglers who still have 11 HP or so each.
Again ... PC win initiative
and all 9 lycanthropes show up in fireball formation? They should have come from all, directions, in waves, several coming from behind total cover. How many HP did the lycanthropes have? A CR 16 monster should have a around 200 HP or more. How did 2 fireballs take them down so far? Even maxed out, a fireball will only do 48 points of damage assuming they all failed their save.
In addition, this sounds like the only fight for the day, or perhaps the second?
My take: Didn't know until reading DND Beyond discussions with designers that isn't spelled out in the book that a sorcerer's twin spell can only be used for cantrips. I will get resistance bringing this up in my group of rules lawyers. But that could've prolonged the combat "maybe" one more turn. There were 5 enemies left who still had a few HP. More than likely, barbarian would've killed two of them on the next round, Rogue killed one on her turn. And then the ranger killed the next two on his turn. In short - it wouldn't have mattered significantly.
The wand of reverse gravity has to go. Period. But it still feel like we're missing something. For example in the first fight the CR 8 wizard in the official rules has 90 HP, I don't know how they're being dropped to 4 HP in a single round by the ranger. For what it's worth, I give NPC wizards more useful spell like Shield, and frequently assume they have mage armor up at all times. There's always maxing out HP of course.
My alternate CR calculator is attached. I'm sure you could do the calculations more easily, but it's something I downloaded long ago and it works reasonably well for me even if I have to adjust PC level up or down a bit depending on the group.
Well, next time make it 50 bandits.... better yet make it 50 ogres, give them a challenge and you an opportunity to slap the mage around and break that wand 
