Those are all conditional form changes. You require no condition such as half health or sun for mega evolution. Also it is helpful to directly know where each mega stands in relation to its base or other megas, as you can only have one mega per team and it can be helpful while deciding between two megas.
Megas are conditional too, the requirement is a fuckin mega stone, remember? that thing you put on your gengar before it got banned?
I think it's still A rank. Its checked by a lot of threats. If running a swords dance set, it isn't that difficult to out speed and ohko with anything that uses an ice type attack. If it use a scarf, it generally locks itself into something that can be easily walled such as eq, dragon claw, or fire blast and then loses significant momentum for the team since it can't run u-turn. Still really versatile like you said, but it isn't unstoppable and it hasn't really gained anything new to make it better than last Gen IMO.
Let me tell you the amount of checks and counters of the 3 'good' offensive sweeping chomp variations,
not even the mega form
Base/Original Set: SD aqua tail chomp in the rain
Yache Chomp:
Checks: latis & scarf dragons
counters: I'll leave that one for the end of the 2nd set since their power level is the same
Haban Chomp:
Checks: strong ice shards, weavile
Counters:
*Inside of rain calcs*
+2 252 Atk Garchomp Aqua Tail vs. 252 HP / 232+ Def Skarmory in Rain: 170-201 (50.8 - 60.1%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Garchomp Aqua Tail vs. 252 HP / 252+ Def Bronzong in Rain: 192-226 (56.8 - 66.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Garchomp Aqua Tail vs. 252 HP / 0 Def Togekiss in Rain: 308-363 (82.3 - 97%) -- guaranteed OHKO after Stealth Rock
+2 252 Atk Garchomp Aqua Tail vs. 252 HP / 252+ Def Togekiss in Rain: 220-259 (58.8 - 69.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Life Orb variation outside of rain:
Checks: Ice Shards, Scarf Dragons, Weavile & the latis
Counters:
*Outside of rain calcs*
+2 252 Atk Life Orb Garchomp Aqua Tail vs. 252 HP / 232+ Def Skarmory: 147-174 (44 - 52%) -- 74.2% chance to 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Life Orb Garchomp Aqua Tail vs. 252 HP / 0 Def Togekiss: 266-315 (71.1 - 84.2%) -- 68.8% chance to OHKO after Stealth Rock
+2 252 Atk Life Orb Garchomp Aqua Tail vs. 252 HP / 252+ Def Togekiss: 191-225 (51 - 60.1%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
4 SpA Togekiss Dazzling Gleam vs. 4 HP / 0 SpD Garchomp: 234-276 (65.3 - 77%) -- guaranteed 2HKO
+2 252 Atk Life Orb Garchomp Aqua Tail vs. 252 HP / 252+ Def Bronzong: 166-196 (49.1 - 57.9%) -- 97.7% chance to 2HKO after Stealth Rock and Leftovers recovery
Overall, this is just a weak argument imo, it has no counters, it can just switch an item and it losses/gains a whole new group of checks, and overall, it's good. Whether it be Setting up SR, revenging, banded powerhousing, SD sweeping or walling stuff, garchomp is just really hard to stop from doing what it wants for it's team.