There are a lot better of sources of information on Picanha out there. This one was recommended PICANHA! - STEAKS vs SKEWERS - How, What & When, all you need to know!. This isn't a guide. It is a way for me to remember what I did wrong so I can do better next time.
So the first thing I did probably was to cut it wrong. There are a lot of discussion and recommendation on how to cut picanha. A lof it will depend on how you are planning to cook it. For steaks you should cut with the grain, for skewers you should cut against the grain. The reason for this is down to how you are planning to cut them for serving. On the skewers traditionally (according to the video) you cut along the skewer. For me the grain of the meat was all over the place. I tried to cut against it but should have probably just cut it straight in steaks. I also should have cut them thinner.
The second thing was having my bbq at too high a temp. The result of this was a rather large fire. Picanha has a fat cap and this obviously renders and melts as the meat is cooking. There was a very large fat cap on my piece. Between the fat and the high heat the fat caught fire and there was a bit of excitement while I managed the situation. If it does ever happen, best you can do is remove the main fuel source i.e. the meat. Once the is gone let it die down. As always you should have a fire extinguisher or fire blanket. Get one appropriate for the fuel source you are using.
I'm not sure if the last one is a function of doing picanha on skewer or of me doing it wrong but where the meat was touching at the ends, they were not getting seared in the same way. Also in the C shape the inside of the C did not get seared.
Not ruling out doing skewer again but I thing next time will do steaks :)