I think Act 2 was generally less focused the whole time, but it really showed up as an issue in the finale for me.
The one thing I think that really didn't land that could have been super easy to change was the Vi/Jinx ending. It was just entirely too telegraphed and really just not satisfying imo. Vi didn't have a single moment of real agency in the finale, and I hate that. Honestly just a conversation between her and Jinx where Jinx decides to leave and Vi stays with Cait would have been enough instead of the sacrifice/escape vague ending.
But I think the real problem with the season is that the whole show was built on two conflicts: Piltover vs Zaun and Hextech Good or Bad. The finale just handwaved Piltover vs Zaun away with a montage to focus on Hextech. And no disrespect to Jayce and Viktor who I liked a lot, but I think the Zaun stuff was pretty clearly the real heart of the show.
I generally like both Ambessa and Mel as characters, but I think this season would have been better without them. They took too much focus away while not being part of the main conflict at all, and their black rose plotline wasn't fleshed out enough to be worth the time it got imo. Just let them die in the jinx missile.
I think Viktor taking Ambessas place in the last battle as the big bad on the ground, leading the Zaun rebellion (planning to dupe them into becoming the husks in the end, obviously the story would need fleshed out) would have really tied it together better. And then Sevika could have actually had a role in the 3rd act. Plus I think the Jayce/Viktor ending could have had a similar main idea without being so scaled up and Ultron/spacey. Through the end of Act 2 it really felt like they were bringing to two conflicts together, but then they just punted on Viktor's connection to Zaun. Actually, Mel dying in the jinx explosion, Ambessa doing what she did up to this point and being killed in Ep 6 would probably work great.
Overall I still really enjoyed it, even the finale. But like I've said, it felt disconnected/unfocused compared to S1 in the end. And the Jinx/Vi ending in particular felt sloppy, even if I'm happy with the general outcome of Vi and Cait together and Jinx off on her own. The Vander bit in the finale just didn't land at all for me though. Honestly I think that bit bugs me more than everything else I said above.