I personally use my laptop with an external display attached and haven't observed any issues so far.
From my experience, it's better to use gdm, as sddm seems to have a problem detecting the external monitor (more details available [here](https://github.com/sddm/sddm/issues/1558)). Of course it's not a blocker, as it is still possible to log in using sddm. I personally find it annoying that my main display remains idle.