I'll explain. I read a lot of forums. In 6th Edition and early, there were always tons of posts saying "do you think this list is good". Now, there are tons of posts saying "can I even take these things". People appear to have no idea how to build a list in the context of the new edition.
I think a few things are going on here. Before I give my opinion, I'm going to quickly lay out how to build a list so that everyone is on the same page.
You first choose Battleforged or Unbound. If you choose Unbound, you literally take anything you want. You still have to make legal selections in each unit, but your entire army could be composed of Ork Killa Kans if you wanted. You pick your Warlord and you're good to go. If you choose Battleforged, things are a little more complex, but in my mind, still pretty straightforward.
For a Battleforged Army List, you simply take as many Detachments and Formations as you want. You can take any kind. If there are any restrictions, they will be listed in the Detachment or Formation text. As an example, your Warlord can't be selected from an Allied Detachment. You do not have to take a Combined Arms Detachment. You can make a list out of all Formations if you want. This seems pretty straightforward to me. It's a departure from previous editions, but it's not overly complex.
Now that I've given an overview of how making an Army List works, I want to touch on what I think the issues are.
First and foremost is that people hate change. Change is hard because it usually requires at least a little effort. Under 6th Edition a player would have a standard "force org chart" and up to one "allied chart". This is no longer a thing. Again. This is no longer a thing. 7th Edition completely changed how lists are made. There is no such thing as a standard force org chart. When people say this, they are really referring to a Combined Arms Detachment. So, 7th Edition list building is tough because it requires a person to forget everything they know about list making. As a rule, retraining someone is harder than training someone. Existing players tend to have more issue with 7th Edition list building than new players do.
Secondly, GW is garbage at writing rules. If I take a Combined Arms Detachment and then take an Inquisitorial Detachment, they are allies with each other. It's fair to say that they are allied detachments... all lowercase. An Allied Detachment (uppercase) is a specific type of detachment. It doesn't help that allied detachment and Allied Detachment mean different things. It makes it very confusing when talking to other people about list building. Would it have been so hard to call this 1HQ/2Troops Detachment something else? Maybe they could have called it a Reinforcements Detachment. Look at me! I'm writing rules that make sense.
Lastly, the Tournament Scene is a nightmare. The majors tournaments seem mired in 6th Edition. They make house rules that require a Combined Arms Detachment and allow for one Allied Detachment. This not only undermines the intent of being able to play with whatever models you want, but makes a number of Codexes unplayable. You can't fill a Combined Arms Detachment with Codex: Legion of the Damned, Codex: Inquisition, Codex: Militarum Tempestus or Codex: Imperial Knights. You can't fill an Allied Detachment either. They have their own special Detachments. By restricting how lists can be made in competitive play, tournaments create a de facto standard on how "secondary market" events are played. If your local store uses Tournaments as a guideline for how to play, then the players are likely to adopt that standard if the store has an active event schedule. It muscles out certain types of legitimate lists.
To conclude, I don't think anything is going to improve until GW realizes that a large segment of their customer base wants clearly written rules that don't change dramatically and are airtight enough to lend themselves to a rigid, competitive environment. I hope the current Games Workshop gets the message and gives us what we want.