For the most part I've been reasonably happy with Linux touchpads (maybe I'm too accepting of less than a perfect experience?) except for one area: Palm rejection.
When you have a giant touchpad it's absolutely necessary to have good working palm rejection, and I've seen far too many laptops completely fail to have palm rejection in Ubuntu. Sometimes you can get it working again by twiddling the mouse driver (enable/disable xinput for example), but sometimes it just plain never works. I've been all over StackExchange and similar places but on some laptops none of the proposed solutions work. I've opened bug reports, but it's hard for developers to fix it when they can't replicate the problem. Sometimes you only get half functionality, like the cursor won't move but it will still click, moving your text cursor to random places when the palm lightly brushes the touchpad and it registers as a click.
On some laptops I just have to pack in a mouse and disable the touchpad entirely. That's not an acceptable solution. I'm really hoping that Ubuntu 18 avoids some of this nonsense.