Agreed. For me, a clear example of that is alpha striking - killing, or crippling most of the opponents in the first turn before they can even go on overwatch. It’s something that only the player can do and it has a tremendous impact on balancing both ways, because it “hides” the real difficulty of many encounters. As long as alpha striking is generally possible it is very hard to evaluate the real difficulty of any mission.
I don’t see a problem with sniper, as LOS is needed and the damage is comparatively low (usually not enough to one shot kill). I see some situational problems, a lot of them to do with accuracy buffs, quickaim, rage burst, and hellcannons and Scyllas, because their size makes them vulnerable.
OK… I’m going to start a separate topic on Scyllas before I go off topic 
Sirens have the same capabilities as the priests, I believe. In fact, previously I had assumed that priests couldn’t mind control more than one enemy at the same time, but I was wrong - they can.
There are methods of preventing mind control, including not depleting your WPs. In fact, that is kinda the role the sirens have - they punish spending WPs carelessly, because the reason they can mind control 2 soldiers at the same time is because they have low WPs.
The devs have said that the sirens will be capped to 2 per mission, and I think that is all the nerfing they need.
One huge problem is that there is a bug that has always been there that results in sometimes double (if not more) damage being dealt from explosions, and it affects all explosives, those of the player and of the Pandas. This is why sometimes an acid nade from an Arthron is just annoying, and sometimes it’s death in 2 turns. The worst I have had recently is an acid worm that dealt 400+ acid damage (yes, it spread over different body parts, but it killed the guy with 180HPs on the next turn anyway).
Once (if? the devs are aware of it from release…
) it is fixed I would suggest capping Chirons per mission and reducing the damage (in the case of acid, probably something else also needs to be done). My reasoning: what is happening currently is that bombard/acid Chirons are the scariest enemies on the battlefield, and we deal with them with extreme prejudice, alpha striking at all costs, or using electric reinforcement to completely negate damage. We are not seriously attempting to deal with them tactically and don’t know if it’s actually possible. Make them a little less scary and see if they are manageable.
Agree on the skills, though as I said before most of it is contextual/due to other OP holes (like accuracy/mobility/damage buffs) and skill spamming.
One important caveat for all balancing that involves nerfing player’s capabilities, and particularly skills: I think they are fine as they are now for a casual difficulty level.
In this regard, I really liked @Wenlock post,
I recommend reading the whole thing, I’m just making a random quote to point to it.
PP can be played in many different ways, almost as different games, and it is being played that way right now. Different difficulty levels should be used to allow for that, to avoid balancing necessary for some “game modes” cutting into other players’ fun.