1. The time commitment issue.
2. It allows a company to give a bunch of "interviews" (assignments) even for candidates they aren't serious about. It can waste a lot of candidate time.
3. There is so much cheating on these that you really can't use it for evaluation. It can really only be a screening tool (which makes 4+ hour assignments really unfair).
4. It primarily evaluates current skillset, not how good they'll be with a bit of training. It's the latter that you care about.
5. They're generally given with little thought about what the company really values. It's typically "uh, gee, we do iOS, give them an iOS app." These projects generally aren't very good at narrowing in on very specific things, like problem solving skills.
6. You don't get a lot of context about why the candidate did things a certain way, and how they would have done things different with some guidance. 7. The results vary dramatically based on how much time a candidate spent on something.
... among other issues.
#1, #4 and #7 are issues with traditional interview as well.
#4 It's incredibly difficult to accurately assess potential during an interview.
#1 and #7: This holds true for white-board interviewing. Most successful candidates (including myself) invested time to practice coding interview questions. I did over 150 questions on LeetCode and it dramatically increased my interview skills. But I guess the upside is that these skills are transferable across companies who do algo/ds interviews so my prep time isn't "wasted".
I think #3 is the only thing thats inherently an issue with the project/assignment interview.
You think you hate whiteboard interviews? Imagine having to to 15+ different take home assignments for different companies before you even get a chance to talk to a person.
#2 -- yes, this is about the hiring manager. But as the candidate, you don't really know what's going on. So it really is specific to the homework interviews.
#3 -- This is a really, really big deal. You're trying to assess people based on homework... that might actually have been done by their buddy.
#4 -- Less true about coding/algorithm interviews. These are focused more on intelligence/problem solving, which is getting more at potential.
#5 -- Absolutely this is about the hiring manager. But it's also very difficult for a homework project to focus on one things (like problem solving skills). The cheating + lack of discussion makes this hard.
#7 -- Time spent makes a MUCH bigger difference for projects than for whiteboard interviews. A 1 hour project vs 20 hours will look very different. A bad candidate with 200 hours of prep will still be worse than a good candidate with 0 prep.