yeah, flicker should be relatively end game viable. def early-mid maps. Not sure how great it is vs shaper. but you aren't talking endgame-endgame.
poe has 2 "endgames".
post story maps. t1-t12. white and yellow maps. This is normal player endgame. character level 85 or so.
Then, there is the actual endgame. Red maps, shaper, guardians, uber atziri. possibly uberlab. char level 90+. This stuff is well beyond the average player. time, currency, etc. to the point people do not even assume 8pt ascendancies, etc when giving out builds. same reason a build guide is usually 90-114points or so, and NOT 121. you are NOT going to get those last 15 passive points for level 85-100.
then that is kindof where "league starter builds" comes in. where streamers etc recommend. They are builds with focused power on leveling and 1-85. They may not scale into the real endgame well. but, are great for farming currency, to set up that end game reroll.
Been a few leagues since I did flicker. I am not 100% where it stands nowadays. 50% crit should be keeping you up yes.
#1. what is your accuracy? your 50% crit is not actually critting 50% of the time. Chance to crit is calcuated twice, and reduced TWICE via accuracy.
50% crit vs 80% hit. =20% miss, 80% hit. only 80% of the hits can crit. Roll acc again. 20% of the crits miss, and only hit."
actual hit, miss crit. 48% hit,20% miss, 32%crit.
you want 90% hit at least, in a crit build.
#2. stunning. possible. not sure what your build is. Yes, while it may look like you aren't getting hit while bouncing around. you are getting hit during flicker. Chance to stun, and be stunned is a factor of how much damage you get hit or hit by. So, having MORE life solves this issue. or various stun immunity items. A real problem from ES/CI builds, as low life=stunned aLOT. Typically flicker is life, and shouldn't have much problems with getting stunned. I can't say I ever actually had issues with being stunned while flickerstriking.
Stun - Official Path of Exile Wiki
#3. not likely. hits are being registered. lockstep might help though.
#4. yes. this is a problem.
#5. mana. this is probably the biggest reason beyond running out of frenzy for stopping a flicker,mid flight. are you sure you aren't running out of mana?
looking back, I don't see a build linked. b ut do see.
ok, yeah. if CI assassin. that might be the problem. First, yes, you are getting stunned.
I wouldn't have recommended CI flicker as a league starter.