From ArchWiki

Regarding the language support on login

The instructions do no always work ( and I got this too) but there is a work-around

"Alternatively, if the above doesn't work for some reason, you could edit the file /usr/share/sddm/scripts/Xsetup and add the command setxkbmap -layout ISO, e.g. setxkbmap -layout se for a Swedish keyboard." I don't know if you want this included or if we are doing something stupid to not getting it to work. --Shieldfire (talk) 08:46, 1 June 2015 (UTC)

Regarding SDDM and root login

There is some discussion on root login in SDDM here: Don't know if it's a "feature" or something is broken in SDDM or if it's a bug in Arch (and openSuse).

Regarding SDDM 0.14.0

As the default shipped /etc/sddm.conf was changed some parts of the page have to be changed when 0.14.0 lands in [extra]. Z3ntu (talk) 10:15, 1 September 2016 (UTC)

SDDM loading before radeon module (ATI video driver)

Some update by 11-Sep-19 made SDDM unable to load normally during the boot process. The Wiki page should encourage the adding of the 'radeon' module in /etc/mkinitcpio.conf [MODULES=(radeon...)]. The update that caused the issue is for sure not related to SDDM, whose version 0.18.1 went out on 31-Mar-19. Probably is kernel or mesa related. Nicoadamo (talk)

This is already pointed out in various places, e.g. ATI#Enable early KMS. As you say, it is not related to SDDM. Closing. -- Lahwaacz (talk) 13:55, 21 September 2019 (UTC)
I disagree with 'already pointed out in various places'. The article you linked only mentions how to, but never says it is a condition for a Desktop Manager to start properly at boot. Nicoadamo (talk)
So let's see if more people have this problem. -- Lahwaacz (talk) 07:00, 22 September 2019 (UTC)