As a food, croissants aren't very healthy. Here's why:
They're highly processed food type, meaning that much of the micronutrient value has been stripped out in the processing of the constituents to influence flavor, storage, and ease food preparation.
They are highly insulin release promoting, but because they lack adequate fiber, these carbs hit your system quickly. They are calorie dense, and are rich in the types of fats that seem to do more harm than good - hydrogenated fats, some trans-fats (meaning they aren't broken down and accumulate in your cell membranes, not a good thing to accumulate). Because they are so calorie dense, its unlikely that you'll burn off these fat and carb carlories efficiently.
Moreover, the fat type used is common in processed foods, meaning that if you eat other processed foods, you may be gaining far more fat calories than can be efficiently burned off because you are supplying not one, but two types of energy - carbohydrate and fat - simultaneously.
That fat type used to make croissants also is known to influence inflammation reactions and enhance the rate of free radical formation in cells, particularly in cell energy production sites (mitochondria and sarcoplasmic reticulum). Thats not what we want. It can cause more cellular damage than necessary when we exercise and prolong the rate of recovery. It can also adversely stimulate our immune systems to a hypervigilent state and aggravate allergies.
Excess insulin and fats have also been shown to influence neurological function. It can reduce our attention span and induce mental fog, make us unusually tired and drowsy in the afternoon, and cause rebound cortisol, which can affect the quality of our sleep, at night, when insulin induction occurs frequently (as it does, in carb loading diets). The fatty acds released from consumption of this type of calorie dense food has been linked to changes in neurotransmitter states and depression.
Prhaps the worst aspect of this extremely tasty food is the mixture of insulin spiking, fast-absorbing carbohyudrates and the rich load of fats. This combination reduces the amount of energy you produce in cells (via oxidative phosphorylation), directing the liver to store this dense load of calories primarily as fat. Fat burnng is turned off, fat storage is turned on, and c-AMP production is turned down. Glycogen storage, rather than being stimulated, is reduced. The result is far from that desired.
Croissants are obviously also a poor source of protein. So as far as nutritive value, this is not a good choice as a carb loading food source.