I heard people talk about it all the time but i would like to know why they are bad?
It's not bad, it's a very nice mid game weapon set... and a great upgrade compared to jet harpoons.
People say it's bad probably because:
1. you can get sonic weapons already after your first mission and about half a month to a month of research, i.e. you can have end-game weapons already in February 2040... which is rather bad design (in my personal opinion)
2. compared to "overpowered" starting gas cannons, they are not such a big upgrade either
So, if gas cannons were not so good, and sonic weapons had at least a few pre-requisites... gauss would be very useful.