First would be waiting on funds. Espeacially when you have champagne taste when your on a cheap beer budget.
Second for me is sanding. I hate it. Even though this is my first build I'm doing alot of fiberglass, which I know isn't a great idea, but since I'm in no rush (due to funds) I'm trying it. Being a perfectionist doesn't help. Feel like I do actual work for an hour or so, then sand for days, then repeat.
Third, having everything planned out then realizing you screwed up someway somehow and have to adjust and recalculate everything. Ok, that one just might be me.