When it comes to cheap food products, I suspect the customer will tolerate "dynamic pricing" downward (e.g., happy hour) much, much more than they will tolerate it upward (order at lunchtime, pay an extra $3!)
That reminds me, though....the campus egg roll carts had "dynamic pricing" before it was cool. If I had a class that ended around 4:00, I'd walk by one of the carts on the way home and ask "what do you have left, and how much for all of it?" I often got something like 2 orders of indian bread, 4 eggrolls, and whatever hell else they had for $5 or so. I could even haggle. "That doesn't sound that great. I'll give you $3 for it all."
"Happy Hour" and other previously established dynamic pricing models are also transparent, in that, the regular prices, discounts, and dates and times for those discounts tend to be listed in menus or other signage.
Wendy's "dynamic pricing" implies that the listed amounts will simply change on the fly and be reflected on the digital menus. In other words, you could drive through at 12 p.m. and see a Dave's Double listed for $10, then drive through again at 3 p.m. and see the same sandwich priced at $6.50 without having the slightest idea why.
In my opinion, the former is perfectly acceptable because the customer is allowed to make an informed choice, whereas the latter is devious and exploitative and should be shunned.