for me it was always the sweating, I would get nervous then sweat like a pig under the gown/mask.
In my ICu rotation last year I finally got some intubations that worked, however to start out I swear they were only giving me people with horse teeth and necks the size of virginia hams. Central lines are easier/harder based on the patient, howe well they're tolerating, awake or asleep, etc. For some reason a thoracentesis was always the easiest for me and if I would continue doing any medicine procedure it'd be that... now it's all LPs, which are getting easier (and less pink CSF)