Ubiquity gives you a choice about what you want to do with your partitions. If you use manual partitioning, you can reuse your existing partition layout. This is what I do. I am able to reuse my existing /home as the /home partition for the new install. And I can choose to format that partition or not.
I have reused existing partition layouts on new Ubuntu installs several times with both Ext4 and btrfs filesystems. It works exactly as expected. No surprises and no problems (as long as you are familiar with partitioning, which it sounds like you are). Just use manual partitioning when you get to that step in Ubiquity.
What does ubiquity do if I ask it to replace previous installation of Ubuntu with a new one?
On the other hand, if you are expecting it to automagically reuse your partitions with all the default installation choices, it will not do that. It will install everything to / (root).