Honestly, given how rare vulnerability of any kind is? I'd say "extra effects" really aren't that much rarer than vulnerability to any specific damage type, at least in the 5.5e MM.
Which is a real shame, and IMO is further evidence that monster design has been badly neglected over 5e's run and especially in the 5.5e update. When <5% of monsters have any kind of vulnerability at all, what is vulnerability even really doing? Especially when 5e was supposed to be the edition that made damage output and HP total the primary source of scaling! That should make Resistance at least semi-special and Immunity genuinely special, and make Vulnerability a prime target for designing monsters that require specialized approaches to kill. E.g. it has 400 HP but it's vulnerable to lightning and cold, so bring ways to deal that kind of damage. Or maybe it has only 50 HP as a high-CR creature...but it's immune to acid, cold, fire, lightning, and poison (maybe some kind of mutant Tiamat spawn?), and resistant to slashing/piercing/bludgeoning, but vulnerable to force, necrotic, psychic, radiant, and thunder damage.
It genuinely just confuses me greatly, the way 5e has chosen to go about its monster design. Unlike the class-design side of things, where they were clearly waffling back and forth for literal years on the most basic stuff, when it comes to monster design they've had literal years to get it right and they've still dropped the ball, continually producing fat bags of HP that don't even make use of the explicit official mechanics available to them half the time!