Many times there isn't an easy way to install bearings without using a drift to get them on. That's not an issue, only using a brass drift to put them on is a bad idea. Just trying to make people who may not be aware of it that using brass drifts and open bearings is not a good idea.
I've never worked with trailer wheel bearings so I can't say. But I have worked with bearings and bearing installation, and have used a drift lots of times because nothing else would work. I've had to pull more than one bearing because of brass chips in the works. Usually caused by someone who thought brass would be a good choice because it is soft and won't won't cause any problems.