I've used the Spike's Tactical bottom loader, and the Hahn top loader. I believe top loaders are a safer and a less troublesome design.
With any of the bottom loaders, the mag block is held is place by the mag catch, and the 1 or 2 containment pins.
When those pins get a bit loose, the only thing holding the mag block in place is the mag catch. With loose pins the mag block can also move around and cause mag related feed problems.
This scenario happened to me years ago with a bottom loader with a tons of rounds through it so the containment pins loosened a bit:
Unload & show clear
Press the mag catch to drop the mag, and the mag block also falls out (oh crap I might need that later)
Now you have no ejector cuz the mag block is gone and the ejector is part of the mag block
You rack the bolt as you've done a thousand times (remember no ejector now)
Unlike all of the 223 you've shot lately, there's now a 9mm round still in the bolt cuz you had no ejector
You drop the hammer and congrats ... you've just been DQ'd.
If you buy a bottom loader, just remember to check the pin/screw tightness regularly.
Quick change mag block concept doesn't matter, it's not like we're switching between 223 and 9mm within the same match.