5.6.2f1 and trying to generate those Lightmaps crashed my project... I had to dig up a backup

using 5.6.2f1 and the issue is still there. UnwrapCL.exe is eating away at ~20% cpu usage for over an hour at this point loading the same fbx model and Unity is "frozen"

Checking the Generating Lightmap UVs checkbox for an FBX file that consists of hundreds of objects and millions of polygons will freeze Unity. I waited for more than 24 hours, but Unity still freezes.

Unity Customer Support, please resolve this issue. Please try to create an FBX file that consists of hundreds of complex models and millions of polygons, and import that FBX file by checking the Generating Lightmap UVs checkbox.

I'm on 5.4.0, and this has messed up the project I've been working on for 2 years. I clicked Generate Lightmap UVs, it stalled for 3 hours, so I closed the process. Came back, the whole building I was working with was gone. Project is now just chairs floating in the freaking air. I know I have to re-import the asset...well, it's been showing the pop-up "Hold On" for 3 WHOLE DAYS! Nothing I can do. Had a deadline for end of this week, and now I don't know whether to just walk away and wait, close the program and try again (for the 3rd time), or just begin the whole project from scratch in Unreal Engine instead

Still happening in 5.3.5p8 !!!

Was this really fixed? I don't mind leaving my laptop overnight, but it's ridiculous that model which has 120k vertices literally took me 5hours for now and I still don't know how much progress was made, anyway, I hope something will be done about this, because I doubt one night will be enough...

I'm having this as well in 5.3.4f1 Even FBX's as small as 2 or 3 MB are taking over an hour and sometimes just plain lockup and never recover. Often I think it's still going and just wait and it's still hung overnight. Sometimes I'm about to cancel and give up and then it's done. We seriously need more of an indicating it's still alive because whether it is actually working or hung up, we have no indication inside of unity of the progress and windows itself is useless as it always reports not responding, even in cases where it actually works.

having same issue with 5.3.2. even simple models take forever. i hope this gets fixed soon.

Using 5.3.2f1, having this issue with any FBX. It runs forever, eats up all my RAM (I have 32GB available for Unity), and basically freezes.

I've tested on small models (10K polys or less, up to 200K polys)

Using Unity 5.3.2 Same issue on a rather large Obj imported model. Working on MacBook, OS X Yosemite.
Unity freezes, no progress status, "UnwrapCL" process taking up 100% CPU. Not a memory issue.
Baking scale on already imported model was set to 0.1 before applying.
Global baking resolution was lowered to 2 texels per unit.
When you force quit unity, the UnwrapCL process still exists after restarting Unity. The object from your assets becomes useless. The already imported model in your scene states missing source.
The only fix is to delete all related objects and re-import a fresh copy of the model.