I had the BakFlip FiberMax on my 2014 Lariat. It was installed by an accessory shop and they also put in a tailgate seal. It really only leaked in the back corners at the carwash. My Tremor has an MX4, which I installed, no tailgate seal. Only leaks at the back corners in the carwash.
The only time I went someplace where I worried about the security of the contents in the bed was a trip to Las Vegas where I had to leave my truck parked at a motel during the day. I had rifles in hard cases in the bed, so I chained and locked them to the lash points in the bed. Still, no one bothered my truck or the cover. Yes, one could easily defeat the cover with a prybar or large screw driver.
I also installed the BakBox under the cover in the Tremor. It allows me to stow the stuff I like to carry around, such as tow straps, jumper cables, shovel, etc. In my previous truck, I had to keep this stuff in a bin in the bed or under the back seat. While the BakBox is handy, you do have to open the cover all the way to access it. And, while it folds up, it still sits about 8" from the front of the bed taking up space. If you open the cover all the way to haul stuff, the contents of the BakBox are exposed. It's a compromise.