Asides from them being expendable to Palpatine and Vader, they were still a serious threat to most opponents. We see them obliterated by characters like Ahsoka and Maul as they were not powerful enough to counter opponents of that level.

However, they were still formidable threats to most other Jedi such as Ezra and Kanan. The Grand Inquisitor was a superior combatant to Kanan, but a fearless Kanan instilled so much fear on the Grand Inquisitor that he was overpowered and defeated. But before then, Ezra and Kanan constantly struggled to survive duels against the Grand Inquisitor. They later found the same issues fighting the Fifth Brother and Seventh Sister on multiple occasions.

Basically what all of these people have said. We saw their training in Tales of the Empire, and it’s mostly just mental stuff. Their final test is to determine how efficiently they can use ruthlessness. It’s all about overcoming their upbringing as padawans, and so their skill doesn’t improve. Their training merely helps them use that mediocre skill for harsher purposes.

Certain inquisitors have more experience or trained better. Cal Kestis has difficulty defeating some of them despite being a well-seasoned Jedi Knight by the time of Jedi: Survivor, though others he easily dispatches despite being a padawan in Fallen Order. Some inquisitors take countless battles to best, while others are killed in a single swing (ahem reanimated Marrok).

These explanations work, but god is it lame to have them be such a non-threat against major characters. Much prefer the Legends inquisitors’ narrative role.

They also have some serious plot armor against them in their defense.

^^ Even against non-major characters they kinda suck, 3 of them got their butts handed to them by fresh out of the tubes Clone Troopers who never seen combat. It also doesn't help matters that all of them are basically either dead or abandoned the Inquisitorius. How is anyone suppose to find them threatening if their membership retention rate is extremely poor?

Yeah the Grand Inquisitor is the only inquisitor I can really take seriously. The rest are all just kinda clowns to be honest

All the more reason for why legends is better than canon

^^ Even against non-major characters they kinda suck, 3 of them got their butts handed to them by fresh out of the tubes Clone Troopers who never seen combat.

When did that happen?

^ Happened in issue 17 of Vader's 2017 series. What makes this even worse mind you, is it wasn't a surprise attack. The 3 Inquisitors just stood there, while a Jedi Padawan mind tricked the Clones into doing Order 66 of them. 1 of which was a former Jedi Master(Tenth Brother) and there weren't that many Clones....there were like 12 Clones at best.

Now the Sixth Bro chopped off the Ninth Sis' leg in order to escape and she did kill the rest of the Clones obviously, considering she appears in the Jedi games. But the fact that 3 Inquisitors couldn't just immediately demolish fresh out of the tube, never been in combat before 12 Clones just makes them look terrible, let alone one of them having been a former Jedi Master and it blatantly not being a surprise attack, they had all the time in the world to react.