If I don't remember wrong, KDE Connect needs to be installed on both the devices you need to transfer file/text to/from, with Snapdrop (and PairDrop) you just need to selfhost it (or use the official website) without the need to install anything and they *can *work even when sender and receiver are on different network
If you want to send something to a computer in school or a work PC or something without admin rights.
snapdrop / sharedrop work in browser, without any installation, and that's the point. As much as I hate web apps, sometimes they are your only option.
I agree that localsend is great when you need to exchange files between your devices often, but when you quickly need to send a file to someone's PC without admin rights, snapdrop and sharedrop are a faster way to achieve that.
i think there is no limit (at least on the software side), because it's local network only, so mothing is uploaded to a server but directly to the recipient. i could be wrong though.