(Hopefully, Steve: on the Forum will be by to offer more. He's the 'local expert' on this subject. But in the meantime, I'll share w/you some info.)
Yes, cedar shingle installation is labor-intensive. Hence, it takes a LOT of labor (time) to install them. The GOOD news is they last 50+ years, certainly much longer, if they're taken care of. The sun will 'shorten' their life but 5 hours isn't too bad if it's just Summer sun (ie., not all year long.) Of course, weather conditions and where you live (along with sunlight) need to be factored in here. Cedar shingles do add value to a home (in my opinion) but you may find prospective buyers that DON'T want them. In other words, if you were to get the shingles then move out in 2 years, you may not get your $$$ back in terms of value. 1) Because you would have had to wait a few more years, and 2) the buyer 'played down' their not wanting cedar shingles. See what I mean? Really, value is in the eye of the beholder. As you do point out, remember, there is a maintenance factor with cedar shingles. With traditional asphalt/fiberglass shingles, there really isn't.
A 40 year shingle is pretty heavy. You need to be sure your roof can handle the load, especially if you have 2x4s for rafters. If the roof is 'braced' from underneath, you may be OK. Have the Roofer check that out. As for mold and mildew, yes, it's very common on cedar shingles. In fact, it's common on vinyl and aluminum and asbestos shingle siding too. IF the conditions are right, you can grow mold on a bleach bottle! In order to prevent mold and mildew, you need to understand how it's formed. It likes wet, moist, stagnent air with temps 70 degrees, or higher. If you're able to vary any 1 of these variables, you could reduce or possibly eliminate the formation of mold.
To help prevent or eliminate mold, you need to be sure that air flows around the entire home's 'envelope' unobstructed. This includes looking around your foundation and up over the roof. (It's 3-dimentional ...) Bushes and plants should be no less than 2' from the foundation to permit air and sun back there (to dry out any water.) Tree limbs should be pruned well back from the 'wall line' of the house. (The limbs shouldn't over-hang the roof, or be even close.) Be sure you have good drainage AWAY from your foundation. BE sure your gutter and downspouts are working properly, and that they dont' have any leaks or breaks or 'low spots' in them. Downspouts should drain at LEAST 3' from the foundation, and water shouldn't run back 'towards' the foundation. The landscape around the foundation should be at least 1/4" per foot, (the more the better), over a minimal distance of 3'. The easiest way to check all this out is to put on your boots, raincoat, and grab an umbrella, and take a walk around the house in the next downpour you get. Just watch the water run off your roof, into your gutters, and out the spouts. If you see ANY 'problems', have them fixed. Oh, watch out for lightening!
I'll let Steve:, or others, say more about this. I've pretty much got cotton mouth. My best to ya and hope this helps.