Less convenient and reliable than regular doors, and probably hatefully noisy. Practicality wise you are left with a lower and narrower opening than the regular doors would give you, the roller mechanism means you loose nearly a foot off the top of the door opening and the mechanism intrudes into the load area as well, as the door either rolls up into a cylinder or runs along the ceiling. Either way a lot of clutter, and the runners are pretty dirty and greasy too
Can't imagine them being less secure, as long as there is a decent latch then they are as secure as the padlock you put on it. The handle will probably be lockable, but it will be a pretty feeble lock and the handle mechanisms on roller shutters are usually pretty awful. On the hire vans I use I don't ever expect the handle and lock to actually work. Also if you have loose items, or if the van is well packed then its easy for stuff to snag and jam the door mechanism, preventing you from being able to get the roller up. The shutters are also pretty easily damaged if anything does shift about
Is it ex-post office? they are pretty much the only people I've seen put roller shutters on the back of panel vans and I've never been able to suss out why they do it.