As I understand it, the primary reason we (Lubuntu) migrated away from Ubiquity in the first place was because Ubiquity was GTK, and Lubuntu was going all-in on Qt as much as possible. Calamares is Qt, and fits like a glove into our system as far as theming. It’s also nice and lightweight thanks to the fact that it’s shipped as a deb package and because it uses Qt, and compared to Ubiquity (and also the Ubuntu Desktop Installer) it’s really fast. The other installers have pauses between screens, Calamares reacts instantly. Calamares also has features that are lacking in the Ubuntu Desktop Installer and in Ubiquity (like encrypted manually partitioned setups) that are quite handy for us.
The Lubuntu team did a detailed comparison between the features of Calamares and the Ubuntu Desktop Installer recently, and while the full comparison was never published that I’m aware of, it revealed that Calamares is a very decent alternative to the Ubuntu Desktop Installer - both had features the others lacked, Calamares excelled with better partitioning, better locale customization, less resource consumption, and better integration with Qt, while the Ubuntu Desktop Installer was much better with Active Directory login, OEM setup, proprietary driver installation, and setups involving LVM (although Calamares may soon have powerful LVM features - at the moment they don’t work but they look promising and we want to work on fixing them).
Lubuntu’s current goals for our installer is to bridge some of these gaps - we want better driver installation, better OEM setup, and better LVM. Since LVM is an upstream thing, we’ll probably be able to help get that fixed for everyone who uses Calamares. As for OEM setup and driver installation, both of those things will require Lubuntu-specific customizations (config for OEM setup, and code for driver installation).
If a flavor is GTK-based, and especially if they don’t want to go to the effort of OEM setup and driver installation stuff but still want those features, the Ubuntu Desktop Installer is probably your best bet. It is non-trivial to implement for a flavor from what I hear, but it’s doable (Ubuntu Budgie proved it). If a flavor would rather have something Qt-based and is OK with the initial setup work of configuring Calamares properly, Calamares might be a potential fit.
(My personal favorite feature of Calamares is the detailed partitioning capabilities on the manual partitioning screen. I once did an install onto a machine that had an NVMe drive, but that didn’t have NVMe support in the BIOS. This meant that the kernel and bootloader had to live on a separate drive entirely for the system to boot. So that’s what I did - I put the root partition on the NVMe, and plopped /boot and /boot/efi on a USB flash drive. It worked fantastic, and I daily-drove that setup for months thereafter. I remember believing that the Ubuntu Desktop Installer couldn’t do this at one point, though perhaps I misunderstood, and perhaps it can now, but for sure Calamares could do it and it did it quite nicely.)