@Alicia Marks Estimating rehab has been a problem a lot of my clients have had recently. For example, we had a property under contract at $102,000 (single family residence). ARV was around $130,000. 3 different well known contractors had three wildly different estimates for the same scope of work. (#1- $12,000) (#2 - $24,000) and (#3 - $48,000!). Those aren't even remotely similar; so frustrating and confusing for my client. I would imagine the lowest bid was too low, and would have ran over budget (but it was the only one that would have gotten us enough to make the deal). And the third was probably a contractor that didn't want to do the job, or wanted to see if the buyer was a sucker and could be taken advantage of.
I would say Determining ARV is a close second though. It can be hard to get accurate numbers b/c so many people (EVEN investors!!) are just throwing whatever money they have at a property b/c they are so desperate to get something that they don't care, and it's making the comps wonky in some sub-markets.