I have owned a CB550K for 25+ years in Seattle. I rode it as my daily transportation, rain or shine, for 5 years or so before moving to Arizona. I never had any problems whatsoever except one flat tire and I forgot to turn the petcock on once and wondered why it stalled. It has been at my dad's place ever since, and I go home usually twice a year and ride it when I am back (weather permitting at Christmas), with periodic maintenance, tune ups, fresh gas, and oil changes. During the summer visit, I usually ride it to Portland at 80-85mph, with a stop for fireworks on the way. My bike has been absolutely dead reliable. I've owned a few more over the years, and worked on a bunch over the years, and none have had reliability problems once dialed in.
I owned a CB400F for 10 years or so, and have owned 3 or 4 over the years and worked on several more. Same story as the 500 -- once dialed in, with periodic maintenance mine was dead reliable. Same with a 350F I have owned for 20+ years (until the cam chain gave out, but I knew that was going to happen, it had no adjuster and I sort of wanted to see how far it would go).
I've never owned a 450 twin, but have worked on a few. They are great bikes, but I can't speak to their reliability. Some things are easier -- the actual adjustment for valves -- some things are tougher -- actually getting to the valves.
But the fours are much smoother. The 400F is killer if you are staying in the city or making a few quick jaunts on the freeway -- they'll hold 75-80 just fine. The 550 is nimble enough around the city as well but a bit bigger and heavier, but if you want take a longer trip on the freeway it'll do you better than the 400.
My favorite bike is the 350F, but if all was right in the world I would own a 350F, 400F, 550, and 750, all for different purposes (and at several times over the years I have had 3 out of the 4, and once I had all 4 in the stable). But where I am now it's not possible...