with the naval invasions it really does depend.
i've always been in favour of a fake bridge from devon to flanders but i do understand why people don't like that ^_^
the issue is repercussions, remember everytime you change one thing hundreds of others change. if i turn on naval invasions for france then they focus north, so aragon spreads throughout france more, so spain crushes the moors.
and balanced means, things changing due to events. if france and germany are allied and england declares war on germany i want france to invade england or defend germany. i don't everyone to be at war ALL THE TIME.
and equally i don't want everyone to be at peace all the time.
so i trigger random alliances and encourage this by making the ai diplomats have more movement speed. but then you get upset when i mess with the 'core' files