multiple monitor support? you mean like 6-8 monitors? or just 2-3? I have been using ubuntu for 2-3 monitors setup for 10 years, never had any real problem.
However if you want three monitors lined up side by side, this seems to be ridiculously hard to configure at least in Ubuntu. The monitor drag-drop interface in Ubuntu is the worst I have ever seen. We recently had three new hires and had to go through the traumatic experience of setting this up for each of them.
xranrdr -d :0 --output VGA-2 --right-of VGA-1
xranrdr -d :0 --output VGA-3 --right-of VGA-2
That would line up your three monitors in a row. I've been using it to automate testing of a multi-window Electron app and it works very well. You can do grids, columns, a cross - whatever...
They have decided that they will only do whole number scaling, and Unity (up to 7, Unity 8, now a community project, was to be based on Qt) is based on GTK/Gnome.
Frankly the Linux DE have been crapifying every size the DEs (Gnome in particular) figured they knew better how to do the underlying stuff than X11.
::sigh::
I've never once had a problem like that in Windows and I literally switch my display back-and-forth in Windows 7 at least 3 times a day as I move around.