The "pregnant guppy" magazine was one of the big selling points of the Johnson, since it held 10 rounds, could be loaded with regular M1903 clips, and could be replenished with single rounds when partially empty. Furthermore, the barrel can easily be removed for cleaning, and there is no gas system. The M1 rifle, of course, had none of these (supposed) advantages.
It is indeed short recoil operated, with the barrel moving a bit less than an inch. The overall impression is of a well-worked out design, but with the rest of the rifle rather an afterthought.
There were two problems. The first is that it did not come along until the M1 had been adopted and was in production, and it proved much more susceptible to dirt and sand than the M1.
The big hassle was "political", though. At the time, the M1 had not built the superb reputation it enjoyed later and many people in the services and Congress felt that the Ordnance Department had made a mistake. Then, when the Johnson was actually produced, some of its financial backers waged a guerilla war against the Army, the Ordnance Department, John Garand, and about everyone in sight. In many cases the real attack was on the Roosevelt administration, not on the M1 rifle itself, which many had never even seen.
Ordnance was accused (with some rationale) of rejecting the Johnson because it was NIH (not invented here), but there was more to it than that. Springfield Armory was set up to make the M1, and was about to go into high gear. The Johnson factory could never have matched the ultimate M1 production. Further, another change of rifle with war approaching would not have been a good idea, nor would another logistics support system. In any event the Johnson was not adopted, and the M1 proved to be an excellent rifle.
A few Johnsons were used by the Marines, mainly it seems because Johnson was a Marine, but they did not perform well in combat. More were bought by the Dutch to be sent to the Netherlands East Indies (now Indonesia), but they were not ready before the NEI fell to the Japanese. IIRC, it was some of these that the Marines bought, but many remained in storage and were subsequently sold in near new condition.
The small bayonet was necessary due to the short recoil nature of the action; the M1905 or M1 bayonet made the barrel too heavy and the gun malfunctioned.
Jim