I think it's because many of us have been through the mill before, buying rigs when profits were high, then seeing the hashrate skyrocket well past projections and barely able to reach ROI if at all.
It's a pessimistic approach, but I have been burned many times on miners which should have ROI'd in 2-3 months only to reach the reality that it just doesn't happen most of the time...
The point that I would like to stress here with USAO's post is that he is talking about equipment he WOULD HAVE IN HAND TODAY. Mining today, making money today and riding this surge in coin prices. Of course the ROI numbers would elongate for a miner in hand today as you will see the mass influx of miners, hash rate and skyrocketing diff push out the ROI. That is a risk, but at the moment a very acceptable risk.
That is scenario 1 where Bitmain offers to sell a miner and you buy it today, expect to have it in a few days, and hope to capitalize on getting it early and mining like crazy.
The problem is we are talking about scenario 2. A delivery in mid to late July with a fixed price that you have to pay today for a miner that based on the numbers we can see right at this moment looks like the diff skyrocket is going to seriously put a pinch on anyone who will be taking delivery in July. The mass influx of new miners from Bitmain is going to be putting a squeeze on all those profit projections everyone is having fun making right now. Your chances of a short time frame ROI on a July miner is looking pretty slim at the moment unless we all catch a break and we continue to see this spike in coin prices and/or Bitmain does not flood the market with more miners. That is a pretty big IF.
So please keep the skepticism in context. The context is a miner you pay good money for today, that you won't see until July and we all can run the diff calculator to figure out that the numbers starting to get questionable for a short term ROI. That really is the point and the concern is the forward sale of miners when we just don't know what the numbers are going to look like to determine if it is reasonable to think you might have a decent shot at ROI.
If you are new and are not sure what I am talking about lets just have you do a little experiment. Start calculating the numbers in each day in What to Mine and figure out a wild guess on what you might have been paid today for a 504mh 800watt L3+(please note the coin price at the time of recording the numbers to help put it in context), screen shot it or write it down. Do this on a regular interval between now and the end of July. Then compare the numbers. If you want to share the results of your experiment with us here that would be fun. If coin prices hold flat or decline you will see the effects of diff increases with the spike in hash rate over that time frame and maybe have a little greater appreciation of the more conservative opinions expressed here by some of the forum members.
What you are largely hearing is not opinions to NOT buy miners, but frustration that if you want to buy a miner today and get it in your hands while the iron is hot you can't do it. And locking in prices for miners out into the future is very risky.
As long as people understand that going into the transaction then great, no worries, place your orders now for a late July delivery and get them while you can. You absolutely cannot tell if there will be additional batches of miners made available, when, how many, or at what price. So it really is a gamble either way. Place your bets... And in the words of the great one, Mr Spock, "Live long and prosper".