Indiana- no, it's really easy to for trombonists to slide between two notes, provided those two notes are in the same "partial". Partials start at the following notes and go down a tritone: Low Bb, F, Bb (just below middle C), D (just above middle C), F, Ab, Bb, etc.
So, if you want a trombonist to gliss from, say, A(just below middle C) up to D, they could do that in the D partial. Most short distances are no problem, 'cause there's a lot of overlap. Now that everybody has triggers, most (short) glisses are possible.