FDM glass: https://static.dezeen.com/uploads/2015/08/Glass-Printing_Med... (and sadly this counts as a pretty complex example)
Glassware, not complex at all: http://www.raystoreylighting.com/wpimages/wp71af747e_05_06.j...
Somewhat complex glassware: http://www.adamschittenden.com/laboratoryglasswarecondenser....
You can see the clear differences. Assuming you can get the printer at all, 3d printed glass has layers (and therefore isn't going to contain gases or hold vacuums very well), it's easily 10x heavier, far more expensive, ...