Start at the high end, will it take... years: NO quarters: NO months: erm... NO. weeks: maybe days: Unlikely hours: Ha. No.
So you end up with a range (days?-weeks-months). That's too broad, what could go wrong to avoid making it months long project (well we could investigate X, Y, and watch for Z). What needs to go perfect for it to be days? (well we could... wait, days is unlikely).
Those discussions about the high and the low to get "reasonable" confidence are super important.
The estimation mention I dislike the most is "t-shirt size". There is no clear relationship between S/M/L/XL. At least story points let compare two tasks. If you try to give t-shirt sizes points (e.g. "M = 2*S"), then you might as well skip the t-shirt abstraction and just use story points.