Their sabotage attempts of several bridges in Crimea did not go this well, suggesting that the Ukrainians alone aren't the best at understanding explosives, and their successes like "fly a bunch of FPV drones out of a shipping container" are quite a bit simpler than this. "Intelligence agency" is a spectrum of capability. Suggesting that an intelligence agency that tried and failed to blow up a bride twice was the same as the one that executed a flawless operation against an underwater pipeline is a bit far-fetched.
It's not at all obvious that e.g. the drone attack, which involved >100 drones assembled inside Russia by human operatives who were able to safely exfiltrate, and 5 different storage containers, in a coordinated strike on 5 different airfields hundreds/thousands of kilometers apart and away from the border, is a less complex operation than the destruction of NordStream
Snuggling and launching drones from deep within enemy territory is a much more complicated op than a couple of dudes diving in the middle of nowhere.
Bringing that bridge down is also much harder than blowing up the pipeline, because the bridge is covered by a lot of defenses, and naval drones will always have limited payload (if they want to be fast enough to evade defenses). Dudes performing a dive in the middle of the sea far from the battlefield are much less vulnerable.
Wait, NORD Stream was BEHIND enemy lines? And is as short a span as a bridge (so more easily monitored complicating things). How many people traveled across the Nord Stream pipeline a day that required the operators to be hidden from?