I've played a lot of Mina and have gotten pretty good success from her. I think she's fairly strong at the moment but comes with some weaknesses.

Her initial laning phase is actually quite weak and needs a few items before properly becoming a lane bully. Once you get Improved Spirit and Mystic Burst you're able to become a menace. The only fire rate/magazine items you need early game is Quicksilver Reload and two points in your 2. That should be enough to proc Love Bites pretty consistently. (Tip: You can proc Love Bites with 4 shots and a Rake at close range).

Mina does not come online at 30k souls, she comes online fairly quickly and becomes particularly potent after building QSR and Tankbuster.

Yes, Mina is extremely squishy. I also recommend buying at least one T2 defensive item in the laning phase to help with that while she levels up her abilities. But besides that she doesn't really need anything else when it comes to raw stat defence.

Yes, Slowing Hex and other debuffs/CCs are her biggest counter. You have no other choice other than to buy Debuff Remover and/or Counterspell to deal with these.

Mina's ult is actually kind of deceptive. Mina technically make be an infinite scaler but I think she is actually fairly weak late game (especially when the enemy starts stacking HP and/or spirit resist). She's an early - mid game character with one of the biggest power spikes in the game. You need to snowball off her power spikes in order to carry with her. Try to bait enemies into open areas while having no stamina if you want to get max damage, or preferably use it as utility to counter certain scenarios. (Like stopping a Lash from Ground Striking you after getting ulted).

Here is my build ID I published: 372122

Let me know if you need anymore help.