I just picked up a BOSS GT-6B and A-B'ing the fuzz compared to a LBM I have it's definitely not the same. Even using the GT-6B custom fuzz (provides the most control) you just can't get that same smooth fuzziness, and it's hard to get that same overall scooped sound even with the EQ, and the lows just don't sound as thick and fuzzed. It's in the general ballpark, but not super close.
However, you obviously get a TON of other functionality, patches, multiple expression pedal support, MIDI in/out (which can theoretically provide a ton of functionality like master clock sync with other MIDI devices which is good for synth-centric bands, external application patch control and dumps so you can edit the thing on your computer instead of only on the actual GT-6B, etc.).
Pros and cons like everything else.
I currently have an overly expensive string of stomp boxes, then a comparatively cheap GT-6B, and can A-B them with my BOSS LS-2 which is great for comparing apples to apples.