Dynamo lights are probably more reliable than battery lights, as there's less to go wrong, but it might happen.
How do you make that out? There's far more to go wrong with a dynamo generator than a battery light.
I was talking about the light itself, in a paragraph where I'd already separately considered the reliability of the generator and wiring. Obviously the system is only as strong as its weakest link
[1].
A dynamo light doesn't have a battery compartment, which means it doesn't have a method of opening to access the batteries, which is subject to wear and stress, while compromising the waterproofing (or, in the case of external battery packs or rechargeable lights with non-replacable batteries, electrical connectors subject to frequent mating cycles). It doesn't have spring battery contacts. And it generally doesn't have an afterthought of a quick-release handlebar bracket to go wrong, either (semi-permanent attachment is the order of the day, which usually
[2] makes for simpler, better-engineered, attachment methods like nuts and bolts). And of course there aren't any batteries to fail or simply be insufficient for unexpected conditions.
Which isn't to say it can't fail in ways that are common to both battery and dynamo lights - water ingress or vibration damage to the electronics or optics, for example - just that it's avoided a whole load of other points of failure by having less to go wrong.
That said, the most common battery light failure modes are down to human errors which - while cumulatively significant in day-to-day use - are unlikely to be an issue for long audax rides (eg. not planning sufficient battery charge for a ride, damaging or failing to secure lights properly when removing them for security reasons, or just not bringing removable lights with you). The always-ready nature of dynamo lighting is advantageous on a commuting or touring bike, but irrelevant for something like PBP.
[1] In a dyanmo system I'd say that was the wiring. In a battery system I'd say that was the user.
[2] Emphasis on the 'usually'. There are some decently-engineered battery lights that get this stuff right. Dynamo lights and battery lights with external battery packs have something of an advantage in that they have less mass for the mounting to support.