Yeah, good trophy haul, but the Scottish League is uncompetitive at the best of times. Deserves enormous credit for dragging us out of the doldrums of the early 80s, but at the end of the 80s/start of the 90s, as hard as it is to believe now, Rangers were the biggest, richest club in Britain, with the best stadium in the country (which gave us a huge advantage post Hillsborough) and benefitted enormously from English clubs being banned from Europe post Heysel, which allowed us to attract the top English talent at the time. 4 of England's Italia 90 squad were Rangers players (Woods, Butcher, Steven and Stevens), more than any other club.
Given all these advantages, Souness' success at Rangers doesn't actually tell you anything about his prowess as a manager. He underachieved massively in Europe as well.