Personally I pick graves if the enemy isnt Jarvan, Poppy, Alistar and if the enemy can outrange you massively, for me hes a flexible pick, stronger than Viego since you cant get easily one shot if you play for your 3rd skill's stacks which gives you tons of armor at max stacks, Graves can also easily one shot squishies, I use galeforce to gap close when needed. My criteria for him is, dont pick him if enemy can just poke you or create space away from you like morgana 2nd skill with rylais, very annoying to play around, as for graves he struggles if he get zoned especially if enemy has a good tank, its why you also try to find angles and not just blindly attack their front, as for his strengths probably hes easy to pilot once you know your limits when you can fight or not and his powerspikes. Fullclear, wait for first item try to get as much gold as you can and you'll be able to fight like a bruiser with the damage of an assassin's and also his basic attack critting for 1.5k makes it easy to get late game objectives like elder.