A few negatives on
Hard Boxes/panniers from my own experience and those of riding partners and from EXTENSIVE reading of ride reports the last 20 years.
1. You may "feel" your gear is more secure when leaving your
hard panniers, but it's really not
hard to break into them. But ... if it makes you feel relaxed ... then that is OK too! To most, they LOOK strong. To a thief, he's already got a screwdriver to pop them open. ... In 30 seconds.
With my
soft bags I use "inner bags", so at night, the inner bags come into Hotel, easy and fast, panniers stay on bike.
Of course you can do same with
hard boxes. Inner bags are good!
Out on the street you have to use more care leaving
soft bags unattended depending where you are. 80% of the time, never a threat of theft. Use caution in Cities of course.
2. Safety again. Broken legs! Most know of the stories of a rider's leg being trapped by metal pannier by fully loaded bike and broken. Or how much it hurts when you're paddling through sand and the pannier WHACKS the back of your leg! OUCH!
Granted, fairly rare thing but it does happen. If you fall with
hard Alu boxes ... get your legs CLEAR of the bike if you can!
3. Crashing
No question
Soft bags are better if you fall, not only because of the broken leg safety issue mentioned above but because of DAMAGE to a metal pannier.
On one trip one of our riding companions
hard metal bags would not shut properly.(he had fallen a couple times) Several Metal Smiths tried to straighten them out without much success. He used a Bungee Cord to secure it. It could never lock.
Heard several similar stories about damaged metal panniers. Pelican Plastic boxes fair better, as do some GIVI plastic cases. Alu boxes make a nice picnic table and are great for placing stickers (the principle reason, IMO, why most like the big boxes better!!!

) but if you fall you always risk tweaking them out of shape, making them UNUSABLE.
4. WEIGHT
This is a big one, IMO, especially if riding off road or Two Up. If you never go off road, then, IMO,
hard Alu boxes are fine. But once I adapted to
soft bags and developed a good "inner bag" system, I've been happy.
When I switched from plastic (very light) E43 Givi boxes to
Soft panniers, I was able to knock off 30 lbs. total! That is A LOT of weight on a 650 dual sport bike!
On an overloaded R1200GS, maybe not such a big deal, but on my bike I could really feel the difference, especially OFF road.
5. COST
This ones obvious if you've shopped around, although the prices for recent so called ADVENTURE
soft luggage has gone WAY HIGH, IMO.
I paid $100 for my current set of Nelson-Rigg
soft panniers. I don't care if they break, tear or wear out. They are expendable. Don't believe for an instant that your $800 panniers won't wear out just the same way ... and
hard Alu panniers (around $1200 to $1800) also can get damaged or ruined over time.