Meh endings are a very common problem with the genre, not exclusive to AoT. It's very hard to find a good Shonen ending that managed to please everyone. : r/ShingekiNoKyojin

Translations & Betrayals

Anime Recommendations Thread - Dollars BBS

Spring Anime Catalogue 2015 – METANORN

What are some reasons someone wouldn't like Attack on Titan? - Quora

Nasuverse Discussion Post – Dragon Quill

The ending to attack on titan is one of the most overhated and

80 Anime Like Attack on Titan

The ending to attack on titan is one of the most overhated and

Naruto 699, 700 – Grand Finale (END) – Random Curiosity

Shingeki no Kyojin - 04 - Lost in Anime

What are the animes similar to Attack on Titan and Death Note? - Quora

Translations & Betrayals

Is the ending as bad as people say? (Anime only) : r/ShingekiNoKyojin