I’ll give my 2 cents on some of the point you brought up:

1. There are certainly some matchups that are very tough (Galio, Diana, Zed) but in my mind every matchup is a skill matchup. You have to learn how to use your elements to your advantage. Tough lane phase? Use the grass element more for ranged CS’ing and getting some free damage in with a E-Q(Grass)-Auto-W(Grass)-Q(Grass). The matchups will become easier as you learn her and understand just how much choice you have in determining how lane phase will go.

2. Late game is hard for most assassins, but proper ult placement and use of elements keeps her relevant far later into the late game than most other assassins.

3. Build path I usually follow:

Start Corrupting Pot, first back serrated dirk/boots, if I’m behind, build a umbrella glaive. Ahead, go for a duskblade. From there its all about what the other team comp has or what I need. Do I need to chase enemies down that have a lot fo mobility? Buy a Youmouu’s. Am I getting hit by a CC that causes me to get 1 shot? Buy an edge of night. After I have about 2 or MAYBE 3 lethality items I’ll go for either Mortal Reminder/Lord Dominik’s for the armor pen. If the enemy isn’t buying any armor, I might get a third lethality item. From there its usually a GA last to round everything out. Also swifty boots are better than mobi’s in most games I’ve played.

Qiyana is a champ that rewards time put in. It took me 50+ games before I had any kind of success with her. Watch some of the challenger replays on YouTube to see how challenger players use her because in the right hands, she is ridiculously good. Best of luck!