Is it legally safe even without a specific authorization? Can I also use the brand logos?
If not, which are the risks in the US?
I only know of some picky folks who said no, it was probabbly best to find that out first.
Amusingly for sales stuff that showed competitors products we of course didn't ask ;) I actually got some legal-ish messages via LinkedIn asking me to take down the competitor information... I suspect they contacted me as I'm the "front end" drone and they thought I'd respond quickly / be fooled or something (they didn't contact anyone else). It was sent by someone who was very much not a lawyer at the competing company.
Told them to take a hike after consulting with legal ;)
I list them, I don't currently have logos but I think it would be ok to show previous client's names and/or logos.
Granted most probably do have a clause that it should only be by permission.
Chances are they might come across it someday and ask you to remove it.
I've only heard of this one time, one of my clients had a request by a company's legal to remove a logo on his site in a clients section, but they didn't ask to remove the company name in text just the logo.
As long as it looks professional I think most companies would be ok with it.
However, I sometimes get outsourced to from ad agencies, and after the NDA, expires, I will add that work to my portfolio. And after a client has not been with me for two years I will add them to my portfolio.
I will definitely use logos. That’s never been a problem.