The M1 Garand's en bloc clip is superior to a removable magazine. In addition to being far more resistant to picking up dirt, water, and debris by virtue of being enclosed, the access structure of an enclosed magazine well is superior.
With a detachable magazine, the magazine must have its release pressed; then the magazine removed; then a replacement magazine must be retrieved; then the magazine must be inserted; then latched in place; and finally, the action must be closed to load the weapon. With en-bloc, the cartridge retainer self-ejects, clearing the action. The soldier may immediately insert the new, loaded en bloc, and the action may be closed. The process is less cumbersome, far faster, subject to less obstruction from fouling, and may be done with one hand in one motion.
Arguing that, for example, the M14's magazine is higher capacity is off-subject; that's a configuration choice, and one that affects the protrusion of the loaded weapon into the dirt when shooting prone. Certainly, a Garand could have been built to take two en bloc clips if a higher capacity loadout was of overwhelming importance - but it was not, and practically speaking, the only thing that made an M14 better at suppressive fire was select fire. I aver that a soldier equipped with a Garand could fire essentially the same number of rounds over a two minute period as one equipped with an M14, if restricted to semi-auto mode.
The difference in the size and power of the ammunition used is likewise off-subject. The only issue is the access structure of a one-step en bloc reload method vs. the multi-step, error-prone reload of a removable magazine, and for that specific issue, the en bloc is superior.
Finally, Garand thumb is a very effective teacher; it is easy, after the first lesson, to understand that the bridge of the palm is to be held against the op rod handle until the loaded clip has latched, and THEN released after retracting the thumb. You can even learn that from others' mistakes; I've never experienced Garand thumb. That's my story, and I'm sticking to it . . .