This is super broken at a glance and should never have passed the sniff test before making it into the UA. rogue doesn't move to gain advantage on a ranged weapon attack (Level3 feature: steady aim). Rogue trades 1d6 from sneak attack & makes that ranged attack disarm or trip the target. couple fights later the GM realizes that they are incapable of using any opponents with weapons and a few after that d&d has turned into only monsters that are incapable of being tripped as usable because this can be done every single round from tier2 on.
Spellcasters do this all the time, so how is it broken that the rogue can?
Rogue:
1) Must be 5th level
2) Must hit first, either with advantage but cannot move or with ally next to foe
3) Target must then make a Dex save (tends to be the easiest save to make on average)
4) Foe is either prone, or disarmed
5) Lasts one round
Spellcaster with Tasha's Hideous Laughter
1) Must be [Edit-1st Level, not 3rd] 3rd level, though more uses at higher levels
2) Does not need to hit first at all
3) Target makes a Wis save (tends to be harder to make than a Dex save on average)
4) Foe is all of prone and incapacitated
5) Foe continues to be prone and incapacitated each round thereafter until they make the save
I've never seen Tasha's Hideous Laughter called "super broken" have you? In fact I often see it called weak due to the save requirement.
It gets worse with Dissonant Whispers, a first level spell that also does damage.
Same goes for Hold Person (Paralyzed so even worse) and Levitate (if you have only melee attacks you're screwed).
Wrathful Smite too at first level. Thunder Wave will send many targets prone at first level.
I am sure others can come up with many more low level save or suck spells.
And ALL of these spell effects are worse than a single round trip or disarm. And I have never heard them described as super broken.
The big difference is of course a spell is a limited resource...but not by much. These are all lower level spells (usable earlier than the rogue 5th level ability) which do more harm than the rogue's ability, and often for a longer period of time. By mid levels many spellcasters could cast them every fight multiple times without a huge draw on their resources. In fact as levels go up a single spell like Hypnotic Pattern is the rough equivalent of many rogues doing this every round.
Just not seeing anything overpowered about this. You're trading damage after you hit for a possible ride effect if they also fail their save. My guess is in use that will be used very judiciously due to the save issue.