Consider that the cable must come up from below at each end.
This from Wikipedia: "Many early cables suffered from attack by sealife. The insulation could be eaten, for instance, by species of Teredo (shipworm) and Xylophaga. Hemp laid between the steel wire armouring gave pests a route to eat their way in. Damaged armouring, which was not uncommon, also provided an entrance. Cases of sharks biting cables and attacks by sawfish have been recorded."
Research into undersea cables had been going on for some time. Bell had experience with shorter cables. There was extensive experience with Western Union telegraph cables, although telegraph is less demanding than voice.
For instance, here is an article on the impact of fishing on cables:
Here is an article about a cable between Florida and Cuba
The transistor was invented in 1948, although it took roughly ten years for it be developed into a commercial viable product. That is, able to be manufactured at a cost less than a tube and reliable enough to be useful. Initial applications were portable radios, though tubes were continued to be used in consumer audio devices for years.
When computers came along, computer makers found that tubes used in audio devices were not reliable enough for high speed digital service. Tiny faults that weren't noticed in audio service would cause computer bit errors. Computer makers developed premium grade tubes where the internal materials were of a higher quality and yield better performance, and also physical placement of the structures were more precise. Tubes were also made under cleaner conditions.
TAT-1 had 51 voice channels. By 1978, TAT-6 was in service with 4000 channels, later expanded to 10,000, and TAT-7 was being laid with another 4000 channels expandable to 10,000. If anything, it's surprising that they kept TAT-1 in use as long as they did since it was so tiny and obsolete.
I'm confident that the repeaters worked up to spec. Western Electric's designs were very, very, conservative.
TAT-1 was retrieved and repaired at least once due to damage from a fishing trawler. Here's a report of a repair in 1959 which took slightly over three days from damage to full repair. The damage was in shallow water close to the Scotland end of the cable and a suitable cable ship was in port nearby.