If riser is done 'good' there is no reason it will cause fire.
And capacitor might be a good idea - it will smoothen voltage fluctuations, assuming it is on the 12v line.
Fail..........
There is this myth that slapping loads of uf about has to be good, because its like magic and can store power......,
In reality it is only 'good' if it is part of a deeper understanding of EXACTLY WHAT a capacitor does and WHY it is there.
An example.....
I had a 'friend' nosing at one of my rig power supplies, his analysis was I had useless capacitors, " no need to have .1uF capacitors across electrolytics, becasue 300uF is far bigger for storing power that it 'swamps' the .1uf (yep.. ok , if I need any finger painting done.. I know who to call)
Considering a Video card can pull several tens of Amps during a peak, all a shitty little ill placed capacitor on the wires is going to do is become a potential source for power-supply ringing.....(amazing fact no 1..... a capacitor acts as SHORT until it is charged up..... the inrush is limited only by the resistance of the connections....)