Panniers are less aerodynamic and will slow you down a bit.
The main advantage is ease of use. All those little bikepacking pouches are a pain in the ass to load and unload. It comes down to how much you need to carry. If you can fit everything into two bikepacking bags, fine. But I see people with 5-7 pouches strapped everywhere like mushrooms on their bikes. At that point it's just ridiculous. Get a pair of panniers and be done with it.
Both systems have their place. I use them both but would never consider bikepacking bags for anything other than rugged off-pavement travel. And even there, bikepacking bags have issues. The high load makes steep downhills tricky and makes even flat surface handling iffy.
Packing them is a pain. There’s no real good way of organizing them and equipment doesn’t really fit all that well given the odd shapes of the bags. There’s a whole lot of empty space in the bags or there are a lot of unrelated items stored together to make the space useful. There’s a whole lot of rummaging that goes on finding items you need. That said, I’ve tried just about every packing method around. Off-road with panniers doesn’t work all that well. The bags don’t stay on well.
Trailers are heavy and make the bike handle squirrelly.
Panniers for road are my favorite methods of carrying a touring load for pavement to mild gravel.
For off-road, however, bikepacking bags work best (with some warts) but I keep using different configurations.
