As @Manziel's answer discusses, the biggest hindrance of autoblock/guide mode style belaying is that lowering or giving slack is a major hassle. Given that top rope and lead belaying will involve these tasks, there are better tools for the job that still feature some form of locking, e.g., Petzl Gri-Gri, Trango Versa, Edelrid MegaJul, Climbing Technology Click Up, Mammut Smart, etc. In contrast, top belaying on a multipitch route will rarely require lowering or quickly giving slack, making autoblock/guide mode a good choice (especially when considering the little additional weight and the ability to organize and eat/drink after leading a hard pitch).
There is one slick trick, however, for using autoblock/guide mode while top rope belaying. I have also seen videos of IFMGA/AMGA guides discussing this technique, but am having trouble finding said videos in the sea of social media (E: here's an example from Cody Bradford--Alpine Gri-Gri ). This trick has limited utility--in decades of using autoblock devices, the only time I've used this trick was when belaying at an ice climbing clinic for novices (=lots of falls/hangs and slow progress upwards) on an absolutely frigid and snowy day (=GriGri is unreliable on icy ropes and I really wanted to belay wearing my mittens). It definitely falls into the category of advanced rope tricks for experienced users.
- Set up your belay device as you normally would for top rope/lead belaying. However, connect it to your belay loop with a long, flexible quickdraw that has locking carabiners on both ends, rather than just using a single locking carabiner. A tripled up shoulder sling with two locking carabiners or a short tether/PAS with a locking carabiner also work well.
- Connect the autoblock/guidemode point on the device to your belay loop as well with another locking carabiner.
- As the climber ascends, take in slack in this autoblock configuration on your harness. Note that the slack in the locking quickdraw prevents you from defeating the autoblock mode. Remember that we should always still control the brake strand in an autoblock configuration.
- When the climber (finally) reaches the top, have them briefly unweight the rope, while you unclip the carabiner holding the device in the autoblock orientation. Make sure you're controlling the brake strand!! You can now lower the climber back to the ground with the device in its normal orientation while extended off your harness, sort of like how we do extended rappels.
Here's a picture of what this setup looks like while in "power-toproping mode." When you're ready to convert to lowering, you would simply unclip the red & gold locking carabiner between the belay loop and the belay device.