I mean… you are right, overall. But in the case of Exec Stance it’s description is indeed very clear. It’s active for 5 seconds, period. If the penance expects more than that, then it’s obvious you’d need to look into the subtalents to find a way to keep it active longer than that. And Relentless likewise is very clear about what it does.
Ofc. I understand it might not be as simple for non-English speakers but that’s not really Fatshark’s fault.
But as said, overall, you’re quite right. The game has absolutely tons and tons of minutiae and details most of which are either found sporadically spread here and there in random forums and Steam guides… or not found anywhere at all. I literally made a google sheets project for the game half a year ago for all the stuff I’ve discovered. 
But afaik that’s also an intentional design choice. At least a short while back when the lead gameplay designer left the company, they said in their farewell letter that seeing people delve into the game to discover and investigate the intricacies of the game, finding surprising little bits and bobs of infromation even this long into release is what they were going for and take great pleasure in. And, honestly, I agree!
I mean it’s not a black & white thing, I was right there with you for a long time after the game got released. Darktide’s complexity & depth combined with its small community, and the relative ease of the endgame means there’s far more info than the community can handle. So finding the real info is a huge issue, especially when you don’t even know to ask the right questions or what to look for, and yet the community is absolutely filled with completely wrong information on top! There are people all over the place spreading complete ignorance and even flat out lies with all the authority of an expert, and newbies have no idea what or whom to believe.
(That other Rending vs. Brittleness thread btw has a small flurry of blocked posts, and I only ever block the toxic and willfully ignorant, so I can imagine whatever argument you think is there probably comes from them)
But even then… as annoying as Darktide’s nuances can be, it’s also come to be a huge part of its charm. How many games today can claim to have the kind of depth where even 1.5 years later people still discover new things all the time? When almost every game no matter their seeming complexity and number of choices always devolves into 1-2 cookie cutter builds with the same BiS gear on everyone, no real choice or variation at all, Darktide’s ability to provide hundreds, thousands of powerful build variations many of which have nothing at all to do with the meta while being even way stronger than that meta, is a literal miracle! I can’t help but appreciate that. 
While I empathize with the pains of newbies, personally my biggest problem is with Fatshark’s own consistency. Like, it’s ok (imo) to leave stuff hidden for us to discover. But it’s not ok to expect us to doubt every single thing the game says, because there’s a good chance some of it is just flat out wrong. Like blessings claiming to work on repeated hit when really they require chained combos (Exorcist - Force Swords). Or talents like zelly’s Thy Wrath be Swift which literally states “Enemy Melee Attacks cannot stun you. On taking Damage, gain 15% Movement Speed for 2s.”, when really melee has nothing to do with anything and the talent works for all dmg from nades to fires to ranged, which makes you an unstoppable juggernaut and is considered one of the best offensive utility talents they have - if you know what it actually does.
But yeah. Game seriously needs both better tutorials and at least some basic codex for stuff like that Power you just mentioned (which btw, just making sure, affects Dmg, Cleave and Stagger… it’s really pog). Speaking of Power, another great case in point for the misleading & wrong descriptions: The Rampage blessing was changed from dmg to Power idk 5-6 months ago, longer? Yet the description is still wrong. The fact that it’s Power, not dmg, is one of the big reasons for why the meta shifted from cleave blessings like Savage Sweep & Wrath into Rampage. Yet how can people even tell, unless they just know?