No, I doubt that running with every footwear is safe. Of course this depends on the level of training you have with that specific footwear, but in general I would not recommend running in high-heels, clown shoes or diving fins. And while you probably can run in slippers or flip-flops I wouldn't recommend that either. The footwear is not made to hold on to your foot, especially not when twisting the feet.
I've ran in sub-optimal shoes before, hiking shoes, leather dress shoes and others. This is possible and it primarily depends on the stiffness of the sole how comfortable the running will be, and while you maybe won't injure your knees when you adopt a forefoot strike in these, compared to a heel strike; you need to take other risks, like slipping, into account. Your ankles might be at higher risk, too, as the stiff sole doesn't attach to an uneven surface as a flexible one would do, which your feet could compensate.
It depends on how you want to assess safety and which kind of injuries you want to look at. If you just look at the issues normally linked with running (bad knees, etc) you might have the same injury rates comparing barefoot running with running in slippers. But running in slippers introduces new risks:
You could stumble over the flexible tip of the shoe if you land in a bad angle, which wouldn't happen either with minimalist shoes or running completely barefoot.
The holder in the from between the toes might cut into your flesh badly, which wouldn't happen barefoot or with shoes either.
You can do a lot of stupid things, without ever getting injured, I have done so myself. But I would not recommend anyone to do the same without giving the warning that I might have been just lucky.