If there are 10 different mysterious objective effects, and 5 objectives on the table that all look identical, how am I supposed to remember what they all do? Maybe it would be easier if everyone played with them all the time, so that I remembered the effects by heart (like with tables like the vehicle damage chart or the deep strike mishap chart)...
It really does get easy once you get used to it. If you have 4 or 5 objectives you may want to mark them with dice or something once they're discovered, but there are only 5 different effects + “No effect”. 1 is the booby trap, 2 is nothing, 3 is skyfire, 4 is re-roll 1s in shooting, 5 is +1 to cover saves, 6 is any unit trying to charge you halves the distance. They’re really pretty simple and straightforward.
They can only be used by a Scoring unit which is holding them. And two (arguably three, because +1 to cover saves can help limit the damage a shooty flyer can do) of the results can help against flyers. I don’t know why anyone doesn’t use them at this point; they’re certainly more balanced and tactical than Warlord traits. The chart’s pretty easy to memorize once you’re used to it; although when people are used to rolling on it, they’ll also have the page number memorized, just like they already do for Warlord traits.
There is one argument that I wish to dispel. Just because a rule is in the rulebook doesn't mean it needs or must to be used. Look at 5th, for example. Everyone always played cover as 4+, even though it could be different based on the terrain pieces. (Razor wire I think was a 5+.) Also, no one used terrain pieces as buildings, they were just always impassible.
Neither of those statements matches my experience. Most things were played as 4+ cover because most of the stuff you commonly see on tables was defined as 4+ on the 5th ed cover chart- forests, hills, ruins, and intervening units all included. I didn’t see a lot of razorwire or chainlink fences on tables, but when I did I always played them as their value from the chart (except on those infrequent occasions that a TO overruled the chart).
Buildings being played as impassible was not uncommon as a house rule, but I certainly played using the actual building rules at Adepticon, Vegas Throne of Skulls, and numerous smaller events.
All that said, I do agree with your core point. But IMO a number of 6th ed rules weren't used initially while people were adapting, and by now should be phased in as people should be used to them.