Seems like still many issues with these updates after viewing Apple Official Support Communities, FCP.Co, and others.
Know we have to some time, but still seems "iffy". We are a 2 suite in-house commercial unit at a TV station. ALWAYS booked full. Here are the specs. 2 x i7 iMacs, 32Ram, either 12 TB Pegasus T-Bolt 1 Array, or 12 TB Pegasus T-Bolt 3 Array (One mac is late 2014, other is Late 2017) Currently on OSSierra and FCPX 10.3.4 All importing from camera chips are AVCHD from a Panasonic AF100 or mpeg4 -Cannon C100 MkII. All working great right now.
Companies that make FCPX plugins are now beginning to move to minimum requirements of 10.4 .