Collective experience shows that it is the combined use of several skills, or the spamming of certain one skill that creates OP holes. It's not the skill itself, it's how it effects stack with something else, or with multiple castings of itself.
For example, x2 damage from stealth from infiltrators results in absurdly high levels of damage when combined with other damage buffs, such as from the reckless skill, a weapon type damage skill buff and berserker's blood lust (and especially all of these combined).
For an example of a skill that has OP effects through spamming, take rally the troops (covered in a different ticket), or electric armor (casting it twice basically renders the whole squad invincible).
For an example of a skill that has OP effects when cast by a build specially optimized for it, consider dash. A soldier with 25 in speed, specialized gear (armor), frenzy and/or blood lust is basically Flash.
For an example of a combination of skills that has OP effects, take rapid clearance cast before adrenaline rush.
Yes, in some cases there are separate tickets for these and other OP combos. However, the problem is that although the proposed solutions might fix that particular combo, they will probably create other OP holes, as has happened with the dash nerf.
The problem are not the skills themselves, it's that there is no limit to how their effects can stack.
The solution is simple: put a hard limit on the effects that can be produced by the skills.
That is, set a maximum of
1) APs that any soldier can have per turn regardless of refunds from rally the troops or rapid clearance (I suggest to set it at 8 APs). (and as a corollary, avoid having any skills that can cause damage during the player's turn without AP cost; after the mind crash and mind control nerf, that's only quick aim when used with 1 AP cost weapons)
2) tiles that a soldier can run on a turn, regardless of dash, frenzy, blood lust, etc. (can be made dependant on the multiple of some average of strength, willpower and speed, corrected by stamina)
3) increase to a stat that can be granted by buffs, so limit damage buffs to +100%, stealth to 85%, accuracy to +50%, electric armor to single use.