In most cameras Auto ISO will kick in if the shutter goes below 1/60th second, which may be fine if you have a 18-55mm lens at 55mm, but what if you're at 18mm? You may be able to hold the camera at 1/20th sec, but the camera would up the ISO to give at least 1/60th, which could raise the ISO from 100 to 320 when it probably didn't need to. :shrug:
Now most cameras are pretty good at the lower ISO values, but that could be ISO 1600 to 5000 which could push the image quality over the edge on most cameras. And that is if they can even go up that high.
Most of the new cameras you can set the max ISO to be used. Some can set the shutter speed at which the Auto ISO kicks in. And a few of the newer camera you can set the max ISO, but also have the shutter speed at which the Auto ISO kicks in match the focal length of the lens, though the setting can be biased to come in earlier or later.
The rule of thumb is that the minimum shutter should be a reciprocal of the focal length, so if you have 70-200mm lens at 200mm, then you should aim for a minimum 1/200th sec shutter to hand old. Now some people say you should factor in the crop factor and add 50%. And then you also have to factor in any stabilisation which may allow you to hand hold at slower speeds than the reciprocal of the focal length. The only way to know what your limits of hand holding at various focal lengths, with or without stabilisation, is to test.
Even though some of these cameras are amazing at high ISO levels, the aim should always be to get the lowest possible imho.
There may be occasions when you don't have the time change the ISO of course, in and out of shadowy side streets and open squares on holiday for example, but to rely on the Auto ISO when you have the time to change it, especially when it is not an 'intelligent' ISO, is lazy imho. :shrug:
Each to there own though.