has it been known in real life for fbi agents to make the bad decisions that have been made in this show:

Season 1. Van Alden letting his partner drive that star witness alone after already getting suspicions that agent Sepso was on the take

Season 4 Agent Knox going around to eli's and talking to family

This season. That de-Angelo sending/rushing Eli and Van Alden into to rob ledgers with only newspaper in moneybag

Historically until 1924 with the advent of J. Edgar Hoover, the FBI which until 1935 was called Bureau Of Investigation (BOI), was characterized by incompetence, nepotism and lack of meritocracy Hoover were also built with the regional offices of the institution (other than a post office) and the FBI investigation began that perfect machine that still is today. Maybe that's because the Bereau is so inefficient in the show.

Van Alden was with the Treasury Department's Bureau of Prohibition.

It's silly to project the FBI of today back in time to the 1920s. It was a totally different beast. No one was expecting the Bureau of Investigation to go after organized crime in the 1920s.

Through the 1950s, the FBI was more of an internal security ministry than a law enforcement entity. It targeted radical anarchists, then Nazi spies, then the Soviet Union's spies in the United States, and the Communist Party USA.