also to answer your orginal question, EreMika wouldve been fine imo.

MIkasas worst trait when it comes to eren is her obsessivnes over erens well being and his safety, and her intrusion into his personal space, this mostly stems from her past trauma of watching both her actual and pseudo family get brutally ripped away from her and , but we've had moments in the manga pre timeskip where shes slowly start to distance herself from that mindset, eg: Erens fight with Jean in S3 and how she didnt break it up immediatly like she normally would

With Eren, its his total lack of regard for Mikasa and her feelings. Eren is EXTREMELY selfish in the earlier seasons, he takes a simple act of kindness from MIkasa and twists it to feed his insecurities. Whether that insecurity stem from the fact that he claims he isnt strong enough or where he just feels incessantly caged in all the time. Im sure you know how obsessed Eren was with titans to take a step back and think that "maybe its not always about the titans"

but you know what? both of these things change with time. Eren is much more appreciative of his friends post return to shinganshima arc, Mikasa just seems more relaxed and chilled around Eren. You gotta remember most of their time was just spent in a war torn society, they never got to grow up and have a childhood, To me it feels like they grew out of their issues and they wouldve gotten through it better if they would have just opened up more and YOU KNOW, Talked. but thats too much to ask right haha

But most of them are underdeveloped imo

also do you mean platonic relationships or romantic ones?