Every wave just gets worse than the last. I don't want to even think about how crazy it will be by the time we get to Amiibo of DLC characters. Wave 1 pre-orders were available for pretty much all characters until the last minute, although I don't think many people took advantage of them. I know I didn't and then had to wait about a week for more Link to turn up at Gamestop. Once Marth, Villager, and WFT were discontinued, things suddenly got a bit crazier, but I don't think it pushed too many more people to pre-orders. That was Wave 2 that helped with that when Pit, Falcon, and Little Mac were all incredibly hard to get without a pre-order (and even Luigi and Diddy were scarce originally). Wave 3, at least in a lot of North America, introduced the store exclusive element and the need to pre-order to have any shot at most of the wave, notably Ike and Dedede which made the demand skyrocket for Wave 4. At least the pre-orders stayed open for weeks though with a few exceptions for exclusives. The Super Mario Bros line actually seemed to have had a reasonable rollout where all characters are still fairly easy to get except Gold Mario, which actually makes sense as a collector's item.
But then we had today's release which could pretty much be described as a fiasco. Jigglypuff had site glitches and then when it was finally available was gone in under 10 minutes. Then Best Buy got the 5 characters up but they were gone in a similar amount of time to Jigglypuff. Gamestop tried to do in-store only but still had their website and internal servers crash, making orders take over an hour, but only artificially lengthening how long they seemed to be available. I think Ness at least had an above average quantity when compared to the non-exclusives though, which is a plus, but also likely means no extras at the locations if you didn't manage to snag one. Tomorrow's wide estimated range for Greninja is likely to be no better either at Toys R Us and will likely be worse after how hard it was to get anything today.
Past scarcity has also affected other regions as US consumers pre-ordered stock from pretty much any country that shipped to the US or was easy enough to get to the US with a forwarding address. I fully expect this to increase in future waves.
I really wish Nintendo would just let stores pre-order first, make that quantity of Amiibo plus some additional percentage and stop this ridiculous artificial scarcity that only benefits scalpers and takes the fun out of getting the characters you like. I know I initially planned on buying my four favorites from wave 1 near release (Link, Samus, Peach, and Marth) and then picking up one or two per payday. Now you need to be prepared to buy anything you might want immediately or pay super inflated prices later.