I see plenty of botrytis, PM, etc. On leaves/plants in FULL sunlight with high UVa, b.........just saying, hence my opinion on the matter. Indoor might have different "effects"? Idk, nutrition/genetics/environment could impact these findings as well....
Yeah, insects sit usually on the bottom side of the leaves to avoid getting touched by strong sun light. Indoors they do the same, most of them incl. their eggs are on the bottom side. With their faceted eyes they can see into the UV range and they avoid getting in contact with it. But to be honest, I'm using DE(1cup/3gal) and neem cake(½cup/3gal) at least in soil so insects are pretty rare in my groom anyways.
Most of the time fungal infection occurs in fall when the suns intensity is much lower. Pretty sure there is no PM or bud rot in high summer when the conditions are relatively dry. Spores are almost everywhere out- and indoors and plants "collect" them all the time and as soon as its gets colder and the spores find good conditions(wet enviroment, temps below 20°C, weaker cell walls or damaged leaves) bam! For outdoors I would recommend to use strains with a known natural resistance. Hollands hope for instance is an old outdoor strain developed for growers living above the 50th degree of latitude and has a natural resistance against pm and other fungal infects ... but only up to a certain point. When cell walls gets weaker it always opens the door and if there are spores the infection will happen either way.
Indoors we remove damaged or wilting leaves as soon as we see them, we lollipop to allow a better air flow and we prune off unwanted leaves. All this helps to keep the humidity lower and make it harder for spores to infect the plants.
If you use UVB every day for up to 6h new spores gets killed pretty fast as soon as they come in. I start using it a week before I see the first flowers. If used only in the last 2 weeks there are already spores deep inside the buds and there is no way to destroy them cause the buds are already too dense. In such a case UV light would have no effect.
Most growers also keep their groom clean and desinfect tents between two runs. Outdoors infected leaves lie practically everywhere on the ground. So the probability of an infection is much higher.
Pretty sure there are more reason which I did not think of.
When I grow outdoors I'm using only automatics now and plant rather a few more plants. Regular strains start flowering later in summer and you need to wait at least until end of september. With auto's I can use the hottest two or three month of the summer for the flowering stage and can harvest in august or earlier. IMO, that's the best way to avoid issues with fungal infections.