I have just upgraded from version 1.2.6 to 1.5.0 and my site https://lebonheurestpossible.org/ is running perfectly. Speed has improved noticeably and the layout is more consistent. For the past 2 years I have received praise for its beautiful design - and indeed its content!
I don't upgrade very often because I need to patch functions.php to modify a few settings that I suggest should be accessible from the theme customization. I contacted the team to advocate these changes.