Years ago (decades, actually) I used professionally applied Krown religiously, as did my parents. For me this was on a 90 Integra. It kept it rust free. As the years climbed, I'd have mechanics come out of the back of the shop to tell me how easy it was to work on etc... Then a big rust blister popped out from behind the windshield trim on the roof - a place that Krown couldn't protect. That was annoying.
Back then I was a starving and inexperienced student for some of the period in question, and my time and location for DIY was very limited. There were things I did NOT like about Krown, which may or may not be specific to Krown.
For starters it seems that it affects rubber - different materials are affected differently. Some door and hood seals expanded, suddenly drooping from their attachment points. Some got sticky. Any wipers that touched the stuff at all wouldn't wipe properly afterwards. Some materials seemed unaffected.
The practice of professional application often ends up drilling holes in the body of the car to access certain cavities. This depends greatly on the quality of the person and their level of laziness. "Better a hole with a plug than a rusted out panel". Well, maybe. I wouldn't want Timmy drilling extra holes in any of my recent cars...
I found that the stuff would migrate around inside the body panels, and would be found oozing out of the car until the next June or July. Everything you touched was oily. It would gather in the spare tire well area in pools. "Today me" would be driven nuts. Of course today me would have cleaned it up every season.
But, it worked, except where it couldn't.
These days I use a variety of rattle can products DIY, most sparingly, and it depends on the scenario. I don't like products that stay gooey and accumulate dirt and salt and hold them against the area trying to be protected. I think most cavities should have a cavity wax, which is like a light cosmoline. I really like what I see when something like Wurth Body Cavity Protection Spray is used - moisture beads and runs away, salt and dirt don't accumulate. I think that cavities with seams at the bottom (think - doors, hoods, trunk lids) can also benefit from a little bit of something goopy that creeps - I've used KBS Cavity spray, or a little Krown or Fluid film sometimes - SPARINGLY. KBS creeps and then sets, Krown and FF are gooey forever. In wheel well arches and underbody I'll use a combination of cosmoline and FF and will specifically clean the area in the spring. Cosmoline is welcome to stay but FF and all of the dirt stuck to it needs to go before it gets warm.
It's also worth noting that my personal vehicles moved to galvanized, seam sealed construction and, in some cases, aluminum panels but the family fleet has Hondas and Toyotas.
What I have learned is that spring cleanup is more important than winter protection. In the temperatures experienced for most of the winter there isn't much corrosive action but come spring and summer it can kick in to high gear. Add to that the fact that road salt and other deicing compounds are hygroscopic, attracting humidity out of the air to keep salty mud build up wet and corroding it can be a nightmare. Ever notice in the winter when the road says wet all day, but the sidewalks and driveways are dry? Except where tires have tracked brine onto the driveway and the tire tracks also stay wet all day? That is what the hygroscopic property does. This is what really rots cars.
I also see lots of folks around here say they avoid any kind of undercarriage wash during the winter. I do the opposite, when I saw the positive effects of it on an ~8 year old Sienna. Without ever having been rust sprayed and barely ever washed, the underside was in clean, pristine condition except for where the spare tire blocked the spray of the touchless car wash undercarriage spray - there the seams were rusting and things were looking rough. Once I started DIYing on that vehicle, I cosmolined all the seam areas and touched it up annually.