I have used HEIF for some time and the short version of what you want to know is this:
Encoding and decoding is processor intensive. On my MacBook Pro without hardware acceleration, photos are not displayed instantly as with jpeg, but could take seconds. This is noticeably if you have lots of photos to go through. Consider this if you want others to view your photos as well.
The quality is much higher than jpeg. Especially skies and surfaces with little color change are much less blocky even with 8-bit sources. And it support higher bits per color without needing a different format. And like jpeg you decide how much or little compression you require.
Avoid converting jpeg to HEIF. Unless you have stored the jpeg at near lossless quality, you will loose even more detail. And storing jpeg artifacts require more space.
Apple Photos does not convert your master files to HEIF automatically. Only if you transfer to a device without HEIF support will macOS or iOS convert HEIF to jpeg. Not the other way round. It should make sense as Photos neither did this before with TIFF, PNG, camera RAW or any other file format it supports.
A HEIF photo is basically a one frame HEVC video. Interestingly you can use ffmpeg or other video software to create a HEIF image, but you still need to put it in the right container by some other software. Also knowledge of HEVC compression settings are directly transferable to HEIF.
Encoding and decoding is processor intensive. On my MacBook Pro without hardware acceleration, photos are not displayed instantly as with jpeg, but could take seconds. This is noticeably if you have lots of photos to go through. Consider this if you want others to view your photos as well.
The quality is much higher than jpeg. Especially skies and surfaces with little color change are much less blocky even with 8-bit sources. And it support higher bits per color without needing a different format. And like jpeg you decide how much or little compression you require.
Avoid converting jpeg to HEIF. Unless you have stored the jpeg at near lossless quality, you will loose even more detail. And storing jpeg artifacts require more space.
Apple Photos does not convert your master files to HEIF automatically. Only if you transfer to a device without HEIF support will macOS or iOS convert HEIF to jpeg. Not the other way round. It should make sense as Photos neither did this before with TIFF, PNG, camera RAW or any other file format it supports.
A HEIF photo is basically a one frame HEVC video. Interestingly you can use ffmpeg or other video software to create a HEIF image, but you still need to put it in the right container by some other software. Also knowledge of HEVC compression settings are directly transferable to HEIF.