I can’t comment from experience, because I haven’t really discussed the UI of Monzo in general.
However, I would place a large bet that some of the “concern” from certain people would be:
• Logos (as @Rat_au_van has just said!)
• Displaying things before they are actually there (what is this voodoo!?)
• The fact a banking app can budget and tell you how much you have left etc (especially when it’s wrong, or can’t work out salary cycles etc - Personally, it gives me less confidence rather than more).
I could go on, but I believe it’s the little things people just aren’t used to yet.
I wouldn’t say it’s a bad thing (although features like “summary” which doesn’t cater for everyone is frustrating and useless unless you fit the criteria).
When HSBC, Barclays and all the other legacy banks update their apps, it’ll be the norm.
For now, Monzo will be seen “ahead of their time”, and a lot of people just don’t like things they aren’t used to.