It has become common to use multi-purpose devices such as mobile phones and tablets in-vehicle as navigation devices due to the widespread adoption and use of these types of devices.
As a general rule, we would not recommend using windscreen mounting systems on larger devices.
As you are probably aware, tablets come in a large variety of sizes. The smaller tablets (such as those with 7" screens) are at the upper end of what would be an acceptable size for windscreen mounting.
For example, an iPad Mini is small enough that it could be mounted on a windscreen in certain vehicles, with regard to the safety points raised in this article. However, a full-sized iPad should not be seen as a device which could be safely mounted on the windscreen.
For larger devices, a whole host of other options are available, such as floor-mounted flexible neck mounting brackets, etc.