After battling both the "H" shaped locking clip and the built-in locking clip from the Maxi-Cosi, I'll take the M-C anyday of the year! I didn't realize that Britax also had a lock-off attached to the carseat.
I think it's just as safe, in fact perhaps safer. The seat belt is always kept in the correct spot, no slipping. So when the car's belts automatically lock during sudden stops or an accident, there's no slippage or incorrect positioning.
I don't know if the Britax is exactly the same as the M-C. The child's seat is reclined, a lever is pulled open which opens a flat press where the lap part of the seat belt goes. Then in the space between the child's back and the seat frame back, you slip the entire car belt in and close the buckle. Everything is in the same position as if there was a person in that seat. Snug everything down, snap down the Internal lock-off clip. And then imo, the best happens -- when you pull the lever back into place, it automatically secures the entire connection by pushing the 'wide press' into the lap part of the car belt, so that carseat is tight in its spot. Imo, the locking clip doesn't really get much pressure on it. It just keeps the belt in the right spot. All this maybe takes 4-5 minutes to do the first time and quicker each successive time. It's easy.
With an "H" clip, there's a minimum and maximum distance from the buckle. Weak fingers need not apply. A pain to do if you have a long stalk to get it just right. It has to fit with the carseat -- not sitting on or against the edge of the car seat. It can take a lot of routing through over and over trying to get the belt tight or loose enough. To me, it needs to be as tight as humanly possible since it's only going through the bottom area of the car seat, if there's any give, the seat can tip forward. (It's a major reason that top tethers are so important, imo) The shoulder part of the seatbelt is nigh on useless because of where it's located. It goes up alongside the seat instead of across it.
There's basically one correct method to attach the H clip and countless ways that are incorrect.
I cringe remembering the rare times I asked my spouse to hook back up the Evenflo Triumph in his own car after he had removed it. No matter how often I showed him, gave him the written directions, walked him through the installation where he was doing it, he was extremely creative in finding ways that the H clip could not have a hope of working properly. It was a complete nightmare. Thankfully, I always stopped him before he drove off with our daughter in his car to check it out. He would be upset that I didn't trust him, but every single time, that car seat was dangerously loose or attached so horribly wrong, the carseat would have been ejected if there had been an accident and the door opened.
LATCH was invented to stop mistakes like my husband continually made with the H Clip. Too bad they can't be retrofited in the older cars we can afford to drive.