And also when you have odds of not making it into CC in the first place through no fault of your own, when before you could plan charges that were sure to arrive.
You certainly can roll good averages with fleet, but there is still the fact that some 20% of your charges are going to stall even if you need a very low number, leaving you on the board to be shot to hell.
The fleet is a nerf
12+d6 is higher on average than 6+rerollable2d6
It's also a relative nerf. The average threat range of fleet units went down while their maximum remained the same. For non fleet units, their average range increased slightly while their maximum range increased massively.
I'm going to point something out though: it's no worse than a Fleeting unit that was charging a unit in cover in the same turn they ran (and seeing as this is where I usually saw objectives, or units that were now huddling in a crater where a vehicle used to be, this was a common thing I've seen before). You fix this by being close, the same way you fixed charging into terrain before. Can you fail? Sure, but with how DANGEROUS some assault units are to your average unit (5 Incubi and an Archon removing whole squads of Marines a turn anyone?) this makes the assault phase less likely to complete nuke portions of someone army effortlessly.
While I'm at it I'm going to make a comment about some of these other "nerfs".
Not able to get bonuses for charging multiple units
: Honestly this just means you concentrate your charges instead of stringing out and hitting two separate units to pull both off objectives. This is still a tactic you can do, you just don't get bonuses for doing it anymore.
No charging the second you hit the board
: Having been hit by Snikrot in a multi charge on turn two, I can understand why this is annoying and can understand why GW did this, but if you were outflanking Genestealers it wasn't common that you got them into combat the second they hit the board just because people knew they where coming (since you have to announce Outflankers and Deep Strikers when you deploy). I think the only armies that REALLY got hurt by this are White Scars. They had the speed to make that outflank manuever very useful, now they'll likely outflank melta to just mess up your tanks instead.
No charging first turn from Infiltration
: Anyone who tries to tell me the Warboss sling shot WASN'T an incredibly dumb tactic probably used it ("Let me throw my expensive HQ choice into the enemy line first turn so he'll die in the next turn!" Yeah, that was a feasible tactic the first time someone did it, but with the number of vehicles out there he'll likely just hit a vehicle and then be shot to death.). I've never seen Infiltrated Genestealers but that was because if they didn't get turn one they would be killed pretty quickly. Really all this does is keep you from over extending your army early and getting killed early on.
No assaulting the turn you step out of a non-Open Topped Transport/Assault Vehicle
: Doesn't affect Dark Eldar, or Tyranids or Orks in Open-Topped vehicles (everything but Orks in Battle Wagons with 'ard Case upgrades). The armies that got hurt by this tend to wear power armor, or play regular Eldar (who are two editions old and have a LOT of other issues anyways) so I don't know why the majority of the Xeno players are complaining about something that didn't nerf them (seriously, the ones who complain about this usually play an army that either doesn't use vehicles, or has open topped vehicles.
: Armies that benefit from this are all over the board (Orks, Tyranid Termagaunts/Gargoyles/Tyrannofex (Heavy 20 anyone?), Dark Eldar with Splinter weapons, Eldar with Shuriken Catapults, Firewarriors, Blob Guard, Marines....you get the idea). Basically if you can get more than one shot when you shoot, this is something that helps you. It doesn't benefit shooty armies, but any unit that gets to shoot. The ones who can't tend to be blenders in and of themselves so they don't need the extra killing power. I'm sure there is a lot of Marine players out there who wish they could have shot a unit of Death Cult Assassins before their entire squad vanished in a single assault phase.
Anything Else I'm Forgetting
: 6th Edition didn't nerf the assault based unit, it readjusted how much damage an assault unit could do in a turn. The amount of killing assault units could do in a turn was borderline insane towards the end of 5th Edition, and one of the most frustrating things you can end up doing is having to pick up an entire unit of models before you can even swing. It's even worse when you get multi-charged, a unit gets wiped out, the other unit breaks from the massive penalties and then was wiped all without even an armor save being allowed because you were hit with Power Weapons.
Pure assault armies are probably not as viable anymore but that's not a bad thing. You just can't base your army on being a big "fuck you hammer" that tears whole units off the board every turn and instead have to balance between firepower and close combat power to make an army more effective.
Here's the general place where I sit on this: the game is more balanced from the elite power weapon-wielding assault units who were becoming more common towards the latter half of 5th Edition. 6th Edition asks you to take risks, be more balanced in how you design a list and not just rely on breaking your opponent's head in during the assault phase. Is there a level of randomness? Sure, but most pure assault units have a way to mitigate this anyways.
Really I'm not seeing the reason for the complaints beyond maybe the Flyers (something that I've seen come up as something people wanted brought in from Apoc on occasion), and I don't really see those as money grabbing schemes as most of the 5th Edition books already had a Flyer in the books (many of which where being actively played as skimmers when they were basically WORSE), and even then I can't agree that the complaints because we have cool models that are finally good.
TL;DR: This isn't 5th Edition. Stop using 5th Edition metrics on 6th Edition. And PLAY the game a LOT before you say something is nerfed. Seriously, just because you THINK it looks bad on paper doesn't make it look bad on the Table. That's a common truth for a lot of armies too (like the Sisters who got a LOT of flakk when they came out but prove to be more solid than forseen).