I had a precious plant that died in a plastic pot today. It was extremely hardy tabebuia heptaphylla. The cause of death was root rot- the plant actually fought for survival quite long before all the leaves turned black and the roots darkened n smelled putrid. I actually think my neglect of signs over two months led to the loss. Lessons I learnt: 1- any pot drainage can get choked so u have to look for signs of poor drainage. 2- the signs of poor drainage can vary from water not coming out of the pot to literally standing in the pot. But in between these two the darkening of surface of your pot, esp if u have compost mixed in your soil is a very imp pointer when u don't literally see your pot flooded with water. The darkening happens bcz of the water standing in the bottom of the pot that slowly pushes up some of the compost particles. 3- terracotta pots give the plant the best chance to survive. Even if the drainage hole is blocked terracotta pots will have water evaporate from all of its surfaces. This is ESP not possible with plastic pots. 4- terracotta pots also leach of salts from inside the pot if the drainage is poor. Not possible with plastic pots and only limited with cement pots.
It's pretty depressing to lose a plant to root rot. It's a mistake on the part of the Gardner
I guess that Tabebuia's are large trees. The lesson is that as keen gardeners we should have very good observation skills. I have also lost Bauhinias (which made me really sad like you!) due to stagnant water. A 6 ft tree died in few days. Bauhinia is a very sensitive tree! Pot drainage problems also occur in clay pots.
As to the white crust formation on clay pots; nothing leaches from the pot material itself, since baked clay is water insoluble. However, salts from the supply water and fertilizers keep on concentrating on the surface from slow evaporation. If a plastic pot has good drainage, the (excessive) salts wash off easily.