My 2 cents: I have been in the game a while. Have done my share of rehab flips - single and multi families as well as wholesaled a lot of properties - single and multi families. I have assigned contracts, double and simultaneous closings, bought without end buyer and then sold without any rehab. Have used end buyers money to fund double closings, used private money lenders, my own money and transaction funding to close deals. I can say the most cost effective way to get it done is assignment of contract. There are a lot of moving parts to wholesaling but when done correctly it can operate as smoothly as any traditional closing - maybe easier. I have no anxiety when dealing with my core group of buyers and probably less stress when I do deal with buyers I haven't worked with before than selling to traditional buyers with inspections, mortgages, and just a unsure attitude.
When done correctly from start to finish assignments go as smoothly as any traditional closing - commercial real estate closing have been using assignments forever and still do. Like anything else attention to detail and being knowledgeable in your field makes all the difference.