They have become popular in the last few years. They are smaller, lighter and thus more portable than traditional mounts. They also don’t need to be balanced with a counter-weight (although they usually allow for a counter weight to increase payload capacity, they don’t need to be perfectly balanced). This all saves in set-up time.

One disadvantage compared to traditional mounts is that they absolutely need guidance, and with a pretty fast guide loop at that (like 0.5s). You can’t do long unguided exposures with a strain wave. But anyone who’s serious about astrophotography is guiding anyway so that’s not really a big deal.