The ability rolls in this game are way too high for most of these and cause a difficulty curve that wouldn't pass in any D&D campaign unless your DM is a sadist. In D&D, if a roll needs to be above 15, it's because that is a path the DM really doesn't want you to take as sit will alter their campaign or, give an easy out for something they would rather be hard. This works great in D&D because the DM can easily fudge the rolls or grant situational advantage/disadvantage depending on how you approach it. This does not work out so well in a video game because you are instead presenting an unsurmountable challenge with no chance of success. A prime example of this happens in the first few minutes of playing when you find a newborn intellect devourer. It gives you an option to wound the creature with a dexterity check. My dexterity was 17 due to bonuses and focusing most of the point buy into that stat. It gives me a +3 bonus and tells me I need to get a 15 or higher. Based off of the mechanics as I've figured them out, this would mean that I would've needed to get an 18 or higher if I didn't have the +3 bonus. Now, this part doesn't affect much and just means whether you fight the nasty little creature or get it to join you but, there are several places where these insanely high rolls are required, one of which involves saving the life of a little girl. I'm not saying nerf all the rolls but, as a general rule, DMs usually set their rolls to be based off of tiers of difficulty. I'll present the tiers and examples of what would fall into those tiers.
Tier 1: Easy - Rolls in this tier tend to be anything 5+ and this is used for simple things that you're pretty much guaranteed to succeed at anyways so DMs will usually just ask if your stat or ability gives you a bonus or a negative. If so, they probably won't even make you roll for it unless they want to see if you get a crit fail for laughs. This is usually for stuff like jumping small gaps or over something or feeling the hand of an orc barbarian fishing in your pockets.
Tier 2: Medium - Rolls in this tier tend to be anything 10+ and this is used for things that you have about a 50/50 shot on. DMs usually toss these out if a result could be interesting for the campaign either way and they just want to see where the dice lead you. This should be your go-to tier for persuasion, intimidation, etc. unless the NPC in question is a staunch believer in their side. In other words, a bunch of bandits robbing a tomb, may be less willing to fight a full party of adventurers if they say something intimidating about bodily harm but, if it's just two level one adventurers and the grave-robbers have you out-numbered, it might go up to a tier 3.
Tier 3: Hard - Rolls in this tier tend to be anything 15+ and are put this way to stop players from meta-gaming or cheaping through content. A lot of players will try to use tactics such as planting explosives behind a big boss fight or intimidating/seducing the big bad. This level of creativity is great from players but, if you spent months planning out a campaign, you don't want them to accomplish their goals too quickly so, you'll make it harder to succeed at their attempts.
Tier 4: 'Impossible' - Rolls in this tier are anything 18+ and are meant to put a hard stop to player's shenanigans. If players are doing crazy crap just to see if they can get away with it, this is meant to put a stop to their actions. Also helps when a player doesn't listen to reason. A good example is a player playing as a skeleton trying to perform for a tavern with a trumpet. Yes, this has happened and, although I pointed out skeletons don't have lungs, he insisted on rolling so I put it at this tier. He got a 20... So, I said he succeeded because he put a bagpipe in his rib cage and up through the skeleton's mouth and used the air from the bagpipes to blow on the trumpet.
As you can see, these tiers aren't set in stone and lots of things can alter what tier the roll falls under. This is one of the beautiful things about D&D, nothing is set in stone. The books even tell you that it is more of a guideline and the DM gets final say on everything. It's for this very reason, this game feels so far from D&D. There is no system for checks and balances. The system doesn't grant advantage or disadvantage depending on how you approach a situation. It doesn't scale an ability roll up or down depending on who is in your party or how close in proximity your are to each other. All of these things make D&D what it is and why it's so fun. You can take 5 different groups of players and put them all in the same exact campaign and I can guarantee each group will have a different story of how that campaign played out because they approached it differently and the dice decided their fate.
tl;dr: Find a way to make the system calculate the difficulty of ability checks based on things like stats, party members, classes and races. Having a set flat number to hit on the dice and subtracting from that based on obscure bonuses is killing the fun of the game.
