Yes I brood outdoors but I aim to do it in spring when temps are more favorable. Last year we brooded in March and it was probably a bit too early for my liking as it got down to about 40 at night. We still had them fully weaned off heat and in the coop at... 34 days old.I've read others say they won't work under 50*, not sure if the instructions say this also, but I've had mine on in heater mode at temps close to zero with no issues.
If we consider a hen can keep chicks warm in below zero temps, this is the same scenario. The chicks jet in and out as they need heat, no matter the temp as long as it's available. My only issue with brooding in cold weather is not letting them get outside as much until they start to feather out, because they need that heat source close by.
So if I do take them outside in cold temps it's for short spurts and I monitor for huddling together (they're getting cold)
@rosemarythyme I believe broods all her chicks outside and has a lot of knowledge with this and might be able to add some helpful tips.
I don't use a heat plate though, I use a mama heating pad which have been reported to run just fine down to the 20s.
Various tips and observations in the article: https://www.backyardchickens.com/articles/short-on-time-recycle-a-prefab-brooder.73985/