If you want to visit places like that, you should at least rent a 4x4 type vehicle... not sure an all-wheel car is going to do the trick... but most SUV's should handle it.
Many of the roads in CR are like that... it's just a fact of life in a mountainous place like this. You'll notice, when here, that when you drive on the 'more traveled' paved roads... that the roadsides are literally lined with houses and small commercial areas (usually a small grocery and a hardware; or other Businesses) because it is a paved road!! The 'real' Costa Rica is generally. down the dirt/stone road that is just off that paved road. Exceptions exist, of course... like the La Fortuna area.
Monteverde gets a lot of visitors; but the money it would take to pave the road all the way 'in' simply wouldn't be possible. Same with many other attractions like Rio Celeste, Eternal rainforest of youth, many of the hotels and resorts nestled into forests or on top of mountains; etc. Also... CR is strict about nature areas and making it easier to get to them would only end up endangering them. Less crowds, less crowd control, litter, etc. And places like Monteverde generally don't get big crowds during the height of the rainy season because they have to close trails and such.
The road that goes between el Volcan Arenal y el Lago Arenal will get you to the other side of the lake... but there is no 'highway' to get you down to Monteverde; just country roads... but it's beautiful; enjoy the drive...
Look, even the Paved roads and highways in CR have 'drop offs' without guard rails... it's just a mountainous country. Many 2 lane roads have single lane bridges; etc. CR isn't a Banana Republic, but you're talking about a country where many of the rural places First got electricity as early as the mid 1980s. Think about it... you were probably in or already out of college by the time they had tv and a refrigerator for the first time. Many of the people in my village still cook outside on a crude wood-fired stove... but have eclectric rice cookers and big screen tv's. Hahaha!!! gotta watch those Tele Novelas...
I've grown accustomed to the unpaved roads... and know that if I venture onto one during rainy season (anytime between May and Xmas) there's a chance that part of the road will wash away. We call that Thursday. But of course... the closer you live to a bigger town or small city or even SJ and it's suburbs and x-burbs... the roads will be much better.
More People = Better Roads
Less People = Bad Roads
I believe that some of the very best places to see the Real CR are also going to be a challenge to get to... but worth it. From mountain vistas to secret beaches (no roads at all!) and refreshing rivers with waterfalls deep in the forest.
There are swimming holes near me that are barely reachable ... even on dirt bike. But we all go!! I'm always shocked at how many people appear out of nowhere when I'm at Las Rocas. Not on the tourist circuit... but unique.
If you want good roads, you'll find them, but so will a lot of other people.
Come on down and get your boots dirty.
Pura Vida