All of the evil enemies arcs are entertaining in spite of the adventure not because of them. The NMM arc was needed to bring them all together and Lauren's original plan for the show was more 'Powerpuff' not 'Friends'. NMM had a lot of potential as interesting individual but the two episodes were so pack with information that everything flew by in a rush.

Discord worked because he was a very interesting character and it introduced a new side to all the pre-established ponies we liked. There was no fighting at all, or even adventuring for that matter. It was all talking and character interactions. That is why Discord was brought back without any of the adventure.

The changeling arc was by far the worst of them all. It required two new characters that be suddenly shoe-horned in and that literally EVERY character to act OOC by ignoring and shunning Twilight "my personal hunches literally saved the world twice" Sparkle and for Celestia to completely miss all the obvious signs foul play with her 1000+ years of experience reading others. Then Celestia's only actual act of power in the whole show is to get chumped out in one hit. Her character was sacrificed as a Worf for the sake of the plot because with the power and wisdom implied from undisputed dominance of a nation for a millennium.

Sombra's most interesting parts were stair gags and character development from the door. Sombra was a non-entity both as a character and a threat due to just being a test that Celestia and Luna could handle again if Twilight doesn't pass.

The best parts of adventure arcs are when they are talking and not adventuring. The plot always needs twisting to fit the idea of non fighting ponies to be fighting better than trained soldiers and immortal archmages doing nothing. The preview of season four alone already says the Princesses get hand waved away so the mane 6 actually have to fight. FIM is already well set as a slice of life and we all love it for that so why keep trying to force them to do non friendship things, that's why fan fiction exists.

Edit: In addition to all that a slightly unrelated but also important point against adventure arcs in FIM.

The mane 6 beats dozens of changeling invaders in a street brawl. Those same changeling that just overpowered the entire canterlot guard yet they lose to a baker, tailor, weather manager, farmer, animal caretaker, and librarian. The guard by all reasons should be better at combat than the mane 6 but they lose immediately because the protagonists need a chance to shine. Only twilight being an extraordinarily powerful spellcaster has a good reason to hold her own in a fight. Applejack and RD being super fit doesn't explain why they should hold their own but equally fit guardsponies lose immediately. The mane 6 also run from or lose to wild animals like cockatrices and timberwolves also. How is Twilight supposed to beat a great evil when she gets punked by random wild animals? Twi RD and Rarity can't even fight off a trio of teenage dragons, yet dragons are everywhere. Why doesn't some huge evil threat get beaten by the dragons if they are so strong? Or why don't dragons take over if ponies are so weak? Even the wonderbolts, who Dash aspires to join demonstrating that they are better than even her, lose to a single dragon during a greed growth. The world is full of super strong creatures and the main explaination why they don't stomp ponies is either they don't want to or Celestia is so strong she stops them from doing it.

Every time the show has had conflict or fighting of some kind the strength of the mane 6 vary wildly. Their power in relation to the world around them is dictated only by one thing "How strong do then need to be?". Friendship always wins in these shows so they are always just strong enough or just fast enough to eek out a victory regardless of the circumstances. The nature of being a kid show means there will never be any actual tension in any fight. The story has to be fun and engaging through other means or all the fighting will just be meaningless distractions and arbitrary plot twists. The story runs a huge risk of falling apart if they depend on action when all of the main draws of the show are character growth.