Monzo in particular are reading their forums so feedback can potentially lead to positive changes. I “moaned” about their new app design while it was in Labs but then switched to another bank when it was formally rolled out and I still disliked it.
Equally, we’re all adults and banks are institutions that make mistakes. Sometimes the right thing to do is engage in conversation and try to remedy the situation. You can be annoyed by something without it ruining the relationship completely.
There’s a time and a place to moan, a time and a place to persue complaints, and a time and a place to take your custom elsewhere 