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Note: I am a gold/silver league player so my knowledge of higher tiers is limited. I will focus on the modern format, but will mention wild cards when fitting.
When Conquerer Jacek was revealed, I was both very excited and a bit sceptical about this card. As one of the new legendary Chaos Legion summoners, this card would obviously be cheaper to obtain than the old legendary summoners, with the benefit of having only 4 level to it, making it simple to bring it to a certain league.
The stats looked promising but with a twist that could make this card bad after all.
Speed is king. We all know that, and we also know how Kitty is one of the best summoners in the game, so the +2 speed on Jacek are a huge statement.
Every single card profits from higher speed, but several cards come to mind that really desperately need more speed in the fire splinter.
Grum Flameblade with his 1 speed becomes viable. 3 speed is totally average, but not an almost guaranteed miss anymore. Besides, one kill and you are at 4 speed.
Tusk the Wide is basicly Grums little brother, also having only 1 speed.
Tenyii Striker has only average speed (3 at silver/gold lvl), but with +2 speed and the dodge ability he could actually become a hard target to hit.
Radiated Brute with Enrage could get a lot of speed.
Countess Sinash helps with +1 speed on top.
Sand Worm could become more reliable hitting its target.
The list goes on and on.
The pierce ability is great for melee and ranged monsters and even can benefit magic monsters if the enemy has void armor.
Grum benefits hugely from this, because small minions cannot longer survive 2 hits due to 1 armor.
All sneak and opportunity minions will also not be countered by a little armor anymore.
These 2 abilities are awesome and good to say the least.
His scattershot ability was creating a big question mark in my head though. Scattershot lets your ranged and magic minions attack a random target while ignoring taunt. This can be a useful, but is it if half of your team has this ability?
Will you be lucky for one game, then target the worst minions the next? Wasn't it usually better to focus your damage on one target?
But then again blast exists, stun could be useful to target backline minions as well, maybe even poison?
Back when I was playing Hearthstone, randomness was frustrating at times, but random cards were by no means bad. The infamous Yogg-Saron even had to be nerfed, because despite his randomness he was usually working in the favor of the one playing the card (he would cast a bunch of random spells, but because many spells would grant you a buff or positive effect it was working most of the time, sometimes hurting yourself though).
Nothing is better than trying cards out on the battlefield, so I went and tried.
The results were... mixed.
I have rented a lot of cards and tried out several different tactics against a variety of opponents now, which led to the conclusion that Jacek is generally a good, maybe even very good summoner, but not a godlike summoner as Yodin Zaku or Byzantine Kitty.
When renting them, I have the feeling that I can use them in almost any situation because they are so good at everything, sometimes even feeling like playing them is almost unfair.
I don't have that same feeling with Jacek.
What he suffers most from is the fact that fire cards are just not that good in comparison to other splinters. I mentioned several card that could become viable with Jacek, because they will get decent speed and that is true to an extent.
But the problem is, that other splinters have lots of cards that are already great on their own, which means the buffs they receive by their summoners make them even greater, while lots of your minions are now just closing the gap, instead of becoming overpowered.
Lets compare some fire cards with Jacek as summoner to some water cards with Kelya:
Grum Flameblade with Jacek has 3 speed. Not great, but not bad either.
Djinn Oshannus with Kelya has 6 speed. That is still a 30% chance to miss, not to forget the fact that Oshannus gets to attack first, alongside his many companions.
Flying Squid is a common card that is often played in the water splinter, having 5 speed with Kelya itself and raising the miss chance by 15% through its blind.
Diemonshark has 5 speed with Kelya and when enraged it goes up to an amazing 8 speed.
Axe Master: one of the best cards in the game in my opinion has 5 speed with kelya while hitting two times with 3 damage.
Pelacor Bandit: 6 speed (+flying).
Phantom of the Abyss: 7 speed (+flying and dodge).
This game is an example of what I just described. Of course my team was not perfect as I was experimenting, but if you watch it you will see that I was not the king of speed in this match and you can feel it.
Sadly, the trend of bad, or lets say not the best cards is continuing with Riftwatchers.
Tinderlock had a lot more speed initially, but was nerfed last minute. I dont think that this was a neccessary, even though I could not play on the test server.
Mordeus is another example. The idea of the card is so nice, and he almost makes the cut. Almost.
Let's compare him to Deeplurker. Deeplurker does everything Mordeus does, but better. It is a common card, costs 1 mana less, has more health, more speed (will kill Mordeus first every time) and has the better second ability. You want your opportunity monster to kill the enemy minions, not stun them. Poison is doing a much better job at that.
All that said, I still would rate Jacek as an A-tier tummoner, while Yodin, Kitty and Kelya are S-tier for me.
Sometimes, you absolutely can humiliate your opponent (even Kelya) like in this example:
Magnor seems to work especially well with Jacek, as he did with Qid Yuff before.
Fire Elemental and Pyromancer worked good, but not as great with Jacek as I initially thought.
Fire Elemental has lots of speed but is easily one shot by any opportunity minion.
Pyromancher is slower but survives a little bit more.
Both absolutely suffer from Headwinds (-1 ranged damage), because then 2 damage only causes 1 damage to the side instead of 2.
Serpentine Spy is probably great, but way to expensive for me.
Halfing Alchemist is a really fun card to play with Jacek, as he throws his halving around. I did not test him enough though to say he is op with Scattershot.
Scavo Chemist is a really nice support card that helps gain the speed superiority.
Xenith Monk seems to work very well in low mana games.
My conclusion for Scattershot is, that it is neither good nor bad. Sometimes you get lucky and are thankful for it, sometimes you aren't.
It makes some cards unviable though in my opinion, such as Supply Runner or Cornealus. They both have good universal effects, but their damage is too low to be thrown around with Scattershot.
You can use Lava Launcher and Countess Sinash quite effectively in my opinion.
Djinn Inferni is too much of a glass cannon as he gets one shot by too many.
Ifrit Rising can work and one shot most enemies, but Recharge is too slow for me and is counterproductive to the whole speed thing.
At last, I want to show that you can also dominate the other legendary summoners as well. Even though these are Wild matches and thus there are more and better cards, I think these are beatiful games shown.
Thanks to 
Conquerer Jacek vs. Grandmaster Rathe:
Conquerer Jacek vs. Yodin Zaku:
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