I am using the Sigma 24mm Art (Canon mount) with the Metabones IV on a Sony A7 mk II. The focus is fast and accurate since the A7 firmware 2.0 update and Metabones IV firmware version 47 update. Before the updates, theA7II focus was really slow with the Sigma as well as my Canon 70-200 f4 L IS. With the firmware updates the focus is as good and as fast with the A7ii as on my old Canon 70D. While adapted lenses work well on Sony A7 cameras, they do not have all the features of native Sony/Zeiss lenses. The recent firmware updates have narrowed the gap between native and non-native lenses, but there are still compromises to features when using adapted lenses on the A7.
Another important factor is the weight. The Sigma 24 Art + Metabone IV is really heavy. If you load up the Sony A7 with a heavy lens and adapter, you may negate the advantages of mirrorless. You will definitely notice the weight difference carrying this combo around at an event. Check the specs carefully for the lenses and adapters you are considering.
Once my Zeiss Batis 25 arrives in a few weeks, I'll sell the Sigma Art.
Sony A7 Mk ii | Zeiss 25 f2 Batis and 85 f1.8 Batis | Sony 35 f2.8 and 55 f1.8 | Canon 70-200 f4 L with Metabones IV
Raleigh, North Carolina, USA