i feel MF is situational ... she is a early lane dominate adc and does well with set up for her ult but she is too easy to interrupt and ult can be canceled. i feel she excels at fights in tight confined areas where the enemy is packed /stacked together so her ult can go through all of them.
in a way twitch is similar to this with his ult but he has assassin/pick 1 vs 1 ability due to his invisibility, which to me is like vayne... if he can get an ambush and take out someone in a fight then he will most likely clean it up nicely. but it takes positioning and timing.
jinx i think is a great team fighter with self peel/zone control
cait in my opinion is great in lane, iffy in mid game/team fight phase and best siege and late. she is suppose to oppress the enemy bot lane and get ahead. and use that lead into mid game. but her ult is really weak in my opinion it gets wasted alot , sometimes it best to use early in a team fight to knock out the hp bar of an enemy squishy/carry , her zone control is great and really good at defending from a siege
kog use to be a monster and if he gets ahead and the team plays the "protect the adc " comp with alot of cc/peel enchantments he can legit 1 vs 5 a team fight its just the meta dont favor him in lane and its too easy to punish him in lane /early with meta champs to make it worth the pick. does great amount of hybrd damage if your team lacks it.
ashe is great at enabling other carries on her team with her slow and ult and aoe damage. if your team comp has a hyper carry jungler (for example kha ) and a similar type mid for her ult and W and slow can enable them to clean fights. and if she gets to a point she can become a semi hyper carry herself.
varus /jihn varus/jihn i feel are great for long range zone and pick potential. however they are very immobile and rely on passives for damage, such as jihn 4th shot and varus blight stacks.
sivir is great adc for team fights for her aoe and her ult allowing to engage /catch opponents but i feel her best feature is her wave clear when you need to be able to match wave clear or clear waves from enemy pushing which can stall a game and allow other champs to scale. ( typically this is why she was so heavily picked in pro play)
trist is great for siege and wave clear, can 1 vs 1 , scales into a hyper carry but her early game can push waves and leave her exposed to enemy ganks bot lane.
ez imo is the worst adc in the game. he is popular but egh
xayah is good her feathers catch people off guard and does a surprizing amount of damage.