I just installed KISS to check it out, this is really nice too! I think niagara has a couple more bells and whistles, but it also could be I'm unfamiliar.

• sliding across app for quick select actions is missing, haven't figured out how to access these yet (ie to jump directly into composing an email or text)(See edit 2)
• inline notifications is a big difference standing out for me, I still need to use the notification bar?
• KISS seems very focused on their search bar which is feeling like a bit more typing. I can tap the circle for an app list but it's on the far side of the phone? (See edit 1)
• Niagara tries to be smart enough to bring apps you'll want to the front homepage, when youll need it. ie connecting to Bluetooth headphones pushes my Spotify to the top. I know KISS doesn't know my habits, but it seems simpler based on history of launches.
• niagara relies on more gestures and swipes
• KISS adding contacts to the home screen is a neat approach, people centric design is good

Overall It's small details though functionallly they seem very close to me. KISS still great and I love it's FOSS. They're doing a solid job of a simple, get stuff done launcher. I don't want to sound like I'm shilling, but Niagara has a free version you could evaluate for yourself

Edit: hmm after digging through the settings I see KISS supports a gesture for opening the app list - however none of the gestures are functional on my s23. Strange..

Edit2: Ah ha! Quick actions are available from the search, and add themselves to the history. I don't love having visible duplicates but it's workable.