"Tender" is a subjective term that generally means a boat heels more than the operator would like. The most proper use of the term is when a vessel heels enough to compromise steering and handling when carrying enough sail to provide the power necessary to make progress to windward. This would be a design defect.
It can also mean a boat like mine that likes to sail to windward at a generous heel angle but has the power and control to do so competently.
The business of boats appearing to heel easily to a certain angle and then stop is not a function of hull form. The ability to resist heeling increases at a nearly constant rate until the deck edge goes under and then the rate of increase drops off. At some point, the ability to resist wind force will start to decrease. What people are observing is the fact that the ability of the wind to heel the vessel drops off as the vessel heels since less sail area is exposed to the wind.
You would probably enjoy reading this and the chapter on heeling arm curves (sometimes called "Wind heel curves") discusses these effects:
http://www.rogerlongboats.com/Stability.htm
There is also a discussion here of what "Dynamic Stability" actually is.
