@BoeroerI had some ideas from this build and my Cipher/Assassin recent run, but I just retested more extensively with Mortar/Blunderbuss and Clear Out to be sure. It's a little funky because it really depends on what hits whom first, and whether there are pre-existing conditions qualifying for Sneak Attack/Deathblows. So essentially there are 2 cases, I'll start with the simple one :
1) If you are in the middle of a fight with targets that already have afflictions that qualify for Sneak Attack and Deathblows, every attack (AoE Clear Out, AoE Hand Mortar, AoE Thunderous Report...) on those targets will get Sneak Attack and Deathblows, and also Assassinate if you are hitting from stealth. Powder Burns still seems to behave a little funky but seems to get everything as well (maybe not Sneak Attack all the time and not Assassinate depending on timing/placement). By the way I just landed a Thunderous Report from invisibility on a poor group affected by Gouging Strike with the Cipher/Assassin and it did 457 DMG to the first target and 400+ DMG to each his buddies. Ouch
2) If you are opening a fight from stealth and no targets have pre-existing afflictions, things are a bit more complicated:
Powder Burns gets Assassinate if it's the first thing to land (i.e. if someone is between your target and you when you shoot). In this case, your main attack and the AoE will be considered out of stealth and not receive bonuses.
If you have Powder Burns on but shoot the closest target to you, your main attack lands first and gets Assassinate and Sneak Attack. The AoE hits and Powder Burns don't get any goodie because they happen afterwards. That is only partially true if your attack is landing a Blind affliction for example, because this enables Sneak/Deathblows for all Powder Burns hits and anything subsequent of course.
The first target hit from Thunderous Report gets Assassinate and Sneak Attack. The other hits in the AoE get only Assassinate.
My previous post about Clear Out is of course still true.
I guess there are a few conclusions to draw from this exercise:
Assassin multiclassing is fun
It is confirmed that weapon AoE effects only get Assassinate but not Sneak Attack by default, however they will get Sneak Attack and Deathblows if the targets already qualify with afflictions. As I had mentioned in the Cipher/Assassin build, an opening Gouging Strike or Debilitating Strike with Hand Mortar (or Devastating Blow with Blinding Smoke 😎) is an amazing set up before something like Thunderous Report!
For Clear Out specifically, Persistent Distraction is of course super strong and with engaged targets, you may sometimes get more AoE damage via Sneak/Deathblows than hitting from stealth (however your ACC will be much lower than in stealth and your build is all about Crits, so it might not be that good on average). But it is a good reason to alternate between stealth attacks and regular melee engagement, which also keeps the fight dynamic and fun.
When you use hand mortars from stealth with Powder Burns on, you should really aim for the closest target and not try to aim for someone in the middle of a group otherwise only the first Powder Burn hit will get all your goodies.
Oh and of course Thunderous Report is totally insane, but we already knew that!
Did I mention that every time those attacks hit, enemies get stacking Deep Wounds DoT?