Alright, so lets have a general analysis of each new drop shall we? (Note that this is all theorymonning since the server hasn't updated to make these drops legal in UU as of yet)
Landorus-I
This is probably the scariest Pokemon to have dropped down in this month's tier shift. It can run many effective sets, but what makes those sets extremely dangerous is the fact that they have completely different checks and counters, which can catch many people off guard and pave a way for Landorus or another teammate's sweep. Landorus's most effective set would definitely have to be the one that
FastFlygon posted. It's a mixed set with the potential to 2HKO everything in the entire game. Only some of the most specially bulky Pokemon check it, but then they risk being crippled by Knock Off or just U-turned out into a check of their own. For example, if Landorus runs Sludge Wave, Snorlax and Chansey check it, but they can be Knocked Off on the switch and then U-turned out into a sturdy physical attacker to dispose of them. Furthermore, there are many different variants of mixed Landorus, so you are never safe until you know its exact set. Sand Force sets are a whole different story and can be seen through Team Preview (although some players would take advantage of this and run Sheer Force sets with Hippowdon as a partner), and they can be checked much more easily with the help of physical walls such as Hippowdon and Suicune. This isn't to say that they aren't dangerous, as they also have the capability to cripple everything with Knock Off or outright 2HKO many defensive behemoths like Vaporeon. Landorus can also run Rock Polish and Choice Scarf sets, but the former has an huge case of 4MSS (although with its counters gone, NOTHING can stop it ;-;) and the latter, while unexpected and fairly effective, faces some problems as many Pokemon are immune to or resist its main STABs and coverage moves.
With that said, I believe the best way to get rid of Landorus is to revenge kill it with something like a Keldeo or Crawdaunt, but then again, Landorus would just switch out and be alright :(
Terrakion
Terrakion is another really scary Pokemon to have dropped to UU, but it isn't as bad as Landorus since it can only run physical sets. However, Terrakion can still run a wide variety of effective sets in UU, which range from Choice Band, Choice Scarf, Life Orb, Rock Polish, Swords Dance, and Double Dance sets. Choice Band, while hitting like a nuke and not having ANY safe switch-ins, can be played around with smart prediction and resists like Hippowdon and the like. Choice Scarf is probably going to be one of Terrakion's more popular sets since it gets really good coverage with just its STABs and has a huge Attack even when unboosted. Life Orb sets, however, I believe to be the biggest problems, as it can switch moves and still hit extremely hard. Boosting sets are another scary sight, but Rock Polish isn't that strong and bulky Pokemon like Slowbro can be used to work around it while Swords Dance is fairly easy to revenge kill (although you're probably going to have to sack something to bring the revenge killer in). Boosting sets also face 4MSS, as without Earthquake, Nidoqueen, Jirachi, and Doublade wall it, and without X-Scissor, Slowbro and Celebi wall it. Double Dancing sets are extremely scary if you let them boost both their stats, but you would honestly have to be a prediction god or facing a bad player for that to happen, and they face even more 4MSS. Terrakion's Justified ability is also great and lets switch in on Knock Offs, but when you consider the fact that most Knock Off users can do a number on Terrakion (Aqua Jet Crawdaunt and Low Kick Weavile), it becomes a little less appealing. All in all, Terrakion is a really threatening Pokemon, but it does have a fair share of checks and even counters in UU, so it should be kept at bay compared to some of the other threats like Landorus and Haxorus.
Jirachi
Jirachi is probably the least threatening Pokemon to have joined UU, but it still is a force to be reckoned with. I don't think that it strictly outclasses Mew like people are saying it to, since it lacks Defog, Will-O-Wisp, and the ability to check Mega Medicham. It can however, run a multitude of other sets much better than Mew. These sets include Choice Scarf, Choice Band, Calm Mind, and a specially defensive cleric. Choiced sets are fun to use because Jirachi has the coverage to use them to great use, but its Speed and power hold it back from being as effective as some other Pokemon in the tier. Calm Mind sets are excellent late-game cleaners, but they have 4MSS and require a lot of support before they can sweep. Finally, specially defensive threats have the capability to wall many dangerous special attackers such as Mega Gardevoir, Latias, and even Keldeo, but Steel / Psychic typing isn't what it used to be and now has weaknesses to Ghost and Dark, which prevents Jirachi from doing well against the likes of Hydreigon. It can, however, provide Wish support and also paraflinch the life out of many Pokemon. Overall, Jirachi seems to be a versatile but fairly balanced Pokemon in the current metagame.
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Also, the Bisharp ban will likely bring Latias to many teams, as it also is a great check to Terrakion and Landorus. Weavile and Crawdaunt are sure to get a lot more use, as they are the next best Knock Off spammer while also revenge killing new drops like Terrakion and Landorus. Hippowdon will also see much more use since it walls the two threats mentioned above. Lastly, Deoxys-D leads and Bisharp checks like Cobalion might decrease in usage, since the former now fears Defog much more while the latter is outclassed by Terrakion.