https://starship.rs/presets/gruvbox-rainbow
You can see that when the segments are empty, they still appear as a 1-width segment, rather than entirely disappearing.
It also makes you configure many things by hand. powerlevel10k has an interactive wizard that lets you design your prompt one option at a time (do you want a nerd font? do you want it one line or two? etc) but Starship makes you manually write escape codes if your preferences don't match one of the presets.
No judgment, but I do wonder what people like about Starship that makes up for these things.