The best thing about this software as I see it from my end of using this bug tracking software for quite some time now has got nothing at all! Yes its free with unlimited features but there are so many other paid ones available in market which have more advance functionalities like team collaboration or project management etc..
So one should compare between different softwares before deciding whether they can afford that much money by investing their hard earned cash into them? If you're looking out an advanced solution without any cost then definitely go ahead otherwise keep searching around because i'm sure u'll find better options if your needs be very specific. We started off solving our problem of keeping track & manage bugs/issues raised against projects we were working upon within short span of just 2 weeks since launch date itself.