FWIW I have mixed experiences with plugs........mostly in the from of garden equipment.
My mower has given me afew problems at times, once or twice purley s/p related. it can give very deceptive sypmtoms that send you in the wrong direction totally.
My other stuff like trimmers and such like I admit pretty much I haven't put a spanner to since i ve had them

.....they all still go 2/3 pull.
The last petrol car I had (AUdi Coupe 5S) I did plugs regularly.....again they can play tricks.....even new ones.
At about 100K, the Audi suddenly decided it was going to drop a cylinder

(

). A quick investigation revealed a dislocated plug lead. i.e. when it came away in my hand I reckoned I had grasped the issue.
It can be worth taking a look!
edit; it is a fact AT, that stuff is really crammed into "motors" now. e.g. I wouldn't do an oil change on my transit.....you just can't get at stuff and you'd skin yourself trying. Have a look at the plugs though......about the one thing now you can get at!