ADC Main: http://na.op.gg/summoner/userName=super%20akro
for the most part this is very dependent on support and jungle ganks.
Support plays a big role. Jungle not so much. Good ADCs keep track of enemy jungle location via timers on jungle paths and vision. ADCs just need to be aware and position accordingly. Good ADCs shouldn't be reliant on their jungle to win lane. As long as your support isn't completely outclassed by the enemy support, you can still make a big impact even if your support blows.
even a cs lead is bad if your cs is low
That's one thing you can work on. Most CS is basically free gold. If you are missing CS accidentally than that's an issue you need to fix. Learn to manage waves and when to bounce it and when to freeze it. Wave management not only allows you to get more CS but also dictate positioning and pressure.
Early game is all about keeping up CS while trying to get enough harass until you are strong enough for an all-in. Once you have a lead, you need to abuse it and keep your lead growing while keeping the enemy bot lane as weak as possible. Freeze the lane near your turret. Now the enemy has to over extend to get CS setting up easy ganks for your jungle while also preventing the enemy from ganking/roaming to stop you. Since you are stronger, you can also zone the enemy away from the creep denying the enemy gold and exp. When the enemy bot lane is not in lane, shove the wave into their turret to quickly deny exp/gold. Leave the enemy tower up so you can have better control of the minion wave. When the minion wave is shoved into their turret, try to start roaming with your support to get deep wards in the enemy jungle. This provides knowledge to your team where the enemy jungle is or isn't and allows you to potentially set up easy picks. Another thing you can do when the minion wave is pushed out is farm scuttle, jg, and/or counter jg. Late in the laning phase, you might even be strong enough to duo the dragon while the enemy jg is dead or top. You can also roam mid and gank with your support and/or jg. These are very easy gank since you outnumber them by so much. Getting Tier 1 Mid turret is much better than getting tier 1 bot turret. Mid turret opens up the map and allows your mid laner to start roaming.
I often feel like the team can engage at certain points but they don't so I go for the poke which sometimes turn out well while other times it leads me to getting caught.
In soloQ, pretend everyone is a bot. You need figure out how each bot is set up to play. Some are aggressive, some aren't. You can't change their decision making, you can only change yours. You need to decide, can I do this solo, do I need to wait for my team to engage, ect. If you go in for poke, can you do it safely by yourself? If no, then don't do it unless you know your teammates are going to follow it up. Everyone makes suboptimal calls, even the pros. Low ranked players do it a lot and for every bad call your teammates make, you've made a couple.
What to do if team don't want to engage?
Set side lanes to slow push, rotate frequently, and force fights at objectives such as baron/dragon.
Late game
Your late game issues sound like problems with positioning. Your goal is to do as much damage as you can while living. If you are too far back, you aren't doing damage. If you are too far forward, everyone will hop on your dick. You need to find the safest target to hit and hit that target at max range. As an ADC, you need to be extremely proficient with orbwalking. This allows to you to do max damage while constantly repositioning during fights. Stay with your team, they won't always peel but sometimes they do. If you aren't with your team, they will never peel. You need to also keep track of enemy abilities, cds, and summoners. Does the enemy Annie have Flash+tibbers? Then you should respect her flash+tibbers range until it's down. Same goes for champs with blinks/dashes/ect.
How to deal with a fed "assassin" in the late game.
It depends on the assassin. Keep your flanks warded, you don't want to be flanked by an assassin. Blue trinket provides a lot of vision from a far. QSS prevents death mark and CC. Steraks helps you live through burst. Maw helps you deal with AP. Pink ward helps you deal with stealth/clones/ect. GA and BV are good items as well. Teammates can help as well especially with exhaust.
Also what is everyone opinion on adc?
Most ADCs still have a huge weakness that is only covered by their teammates. The preseason changes made more ADCs viable and made build paths more dynamic. Overall strength of ADCs hasn't really changed much.