There are just so many parts that can ruin a good run at any moment.
Yeah, I'm noticing this a lot right now. I managed to get gold on all other extreme tracks - a lot of it was, through trials and error, realising what could get me over an obstacle 95% of the time (e.g. at X obstacle, don't lean until the end of the ramp, and then lean forward, then bunny hop on back wheel on that platform) so I have a little pattern to follow when doing the track. Sometimes, there would be an obstacle that I just can't find a reliable method for, so I'll just be leaning forward and holding the gas slightly differently each time until one manages to work.
Inferno IV? There are currently 4 obstacles that I have no method for getting over. Sure, I got a silver medal, but Jesus was that hard.
Barrels? Dammit. First long non-steep ramp with obstacle on the incline? Dammit. That bit where you have to hook your bike around the underside of a platform? Dammit. The very last obstacle? It'll be a good day when I know how to properly make that very last bunnyhop.