The advantage of glass mounts is that they keep the slide flat - thus making corner to corner sharpness in the projected image much more likely.
They also don't suffer from slide "pop" as the slide warms up.
This is particularly important for larger magnifications.
The trade-offs are expense, weight and the challenge of mounting without getting any dust into the sandwich.
Some projectors have lenses that assume glassless mounts. They count on the slide having a slight curve.
It is true that bulbs can be a sticking point, but there are some good internet resources out there.
I have a couple of projectors - a Rollei that I bought used and a Kindermann that was given to me.
I haven't decided which one to keep.
Both are quite simple, but work well.