Wait, you don't feel so?
I have maintained a Nextcloud server for a small business for the last 6 years. I agree when I started using it early 2020 that the ui felt less modern, but after some updates down the road up until now it looks completely like it is from this era. Am I missing something? However, I never complained about the UI neither then nor now.
NextCloud's macOS app is implemented in a way that completely disregards any UI guidelines or just common sense for the platform.
Developers either don't use macOS at all, or don't care.
But functionally it works for our small team.
NextCloud generally appears to use their own design system everywhere, Android apps are also not in Material or on iOS (iirc) in Cupertino. It makes for a subpar experience in general but is consistent.