Bug/insect issues: Integrated IR cameras may attract insects or spiders, which nest/build close to or over the lens of the camera, obscuring the field of view. Some users have reported remedying this issue by using external IR.
We use no integrated IR on external cameras for this reason. We use integrated IR on interior cameras only.
The big problem I've seen over many years with integrated IR cameras are spider webs. This is particularly true with the bullet cameras that have a "hood" over the lens as it give the spiders a working platform.
This problem has gone away with the use of the new, popular turret type cameras. I almost never see a spider web problem with the low-light, no-IR turret. Love them. . .
I would like to add some important information concerning clothing and textiles.
Under infrared light, clothing and textiles (e.g. backpacks, bags, etc.) can be either absorptive or reflective. This has the significant effect of making clothing that would appear as one color to the human eye, appear significantly different under IR light.
Depending on the fabric it may either absorb IR light or reflect it. Absorptive fabrics will appear dark under IR light, and reflective fabrics appear white under IR light, regardless of their actual color. For example, a subject in white light may appear to be wearing a black shirt, blue jeans, and dark sneakers, however in IR light they may appear to be wearing all-white due to the fabrics having reflective qualities.
This can negatively affect a response to a threat as a subject may be mistakenly described as wearing bright-colored clothing when in fact they are not. It is therefore important to educate viewers of IR-equipped CCTV cameras that inferences about clothing color should not be made when viewing subjects under IR light.
Notice how the shirt is black with white lettering under visible light, but under IR light the colors appears reversed. If a viewer were to inform security or police responders that the subject was wearing a white shirt, upon viewing the subject in person they may get dismissed by responders as not matching the description given.
Installers and consultants should be aware of this when specifying IR cameras for customers that may require more positive identification of subjects in dark areas. If the color of clothing details are important then an ultra-low light color camera would be a better solution than one with integrated IR.
I suppose in theory yes, although IR doesn't make everything necessarily blend together through reflectivity or absorption. You would generally be able to make out things like the edges of a shoulder patch or screen printing, unless the subject was far enough away that fine details weren't discernible. Unless everything on the outfit had precisely the same reflection/absorption values as everything else you'd notice something was off.
The military has done fairly extensive research into this field for obvious reasons. You will find that all modern military uniforms, web gear, and other worn accessories are completely IR absorptive so as to minimize signature under IR light. If you really wanted to be nefarious it wouldn't take much to buy a cheap IR camera and test out clothes until you assembled an outfit that was completely absorptive and allowed you to blend into the shadows more easily.
I don't know enough about inks and textiles to comment authoritatively, but I suspect that they would not have precisely the same reflectivity values. If you're putting in enough effort to do something like that, you're probably sophisticated enough to find ways around the cameras entirely.