More accurate would be to say "what Psycho V would speak of this?" since he was nominally in charge of Continental Celts.

Best of course would be to ask to a current EB Celtic Team Member, like Power2the1.

As for Gauls and courage, I believe that EB represents this well. Celts are probably the least steadfast warriors in the field, all other points taken in notice - in fact there's a description saying "despite the Celtic propensity to become easily discouraged when facing determined resistance and to suffer under a hot sun", also not counting that average Celtic morale is also lower than that of the Sweboz and the Getai. Many times I've managed to break an opposite Celtic line with a fierce charge, and you'll be surprised at how much the morale of your soldiers floats with battle, more so than with any other faction I've seen in EB.

The conclusion we can take from, though, is that the Celts were not cowards. They lacked discipline, that's true, and discipline is the deciding factor when the going gets tough and battle eagerness gives on to bitter reality, which would require people to forsake their high bids and stand together at all costs - this is what seems to be lacking in their "ethos", at least from the limited grasp I can take. Else, they are superior warriors, and they are very brave.

The propensity to change sides was really just a mercenary affair than anything within the Celts in particular. It wasn't uncommon for people to switch sides all the time when large numbers of mercenaries were on the field, example being when a large detachment of German mercenaries deserted Sweden after Nordlingen, due to debt, defeat and the likes. It's why mercenaries can be unreliable some times, but that also depends on the commander; we have Hannibal, for example, keeping together a quite heterogeneous bunch of mercenaries for a long time, and many fought to death for him. The advantage of a system of citizen soldiers (continuing our discussion from other thread) is that the citizen soldiers owns more allegiance to the state, and while he may not be the best, IIRC Roman armies had a structure that was far simpler for a run-of-the-mill politician and his subordinates (chosen more for political acumen than military skill) to grasp and conduct than the kind of army Hannibal had to command.