Several ships tried to approach the Borg cube and were defeated by its tractor or cutting beams in "Q Who?", "BoBW", "Emissary" etc. Would they have had more success by going FTL?
Well, the idea is obvious enough - and if ships were so willing to perform suicide runs at STL speeds, surely a few must also have tried FTL, at least after seeing how STL was achieving nothing.
Yet the Borg were not
defeated off camera. So either the things that looked like STL suicide runs (say, the plunge of the Melbourne
towards the Cube without firing a shot in "Emissary") were something else and nobody but Riker was willing to attempt glorious suicide - or then FTL ramming does not work. The odds of it never having even been attempted are pretty low in comparison...