Ditto, visibility. Little things conspire to thwart bike fit and ergonomics. If we tweak our bike fit indoors, we're probably doing so without helmet, sunglasses or safety glasses, caps with visors, etc. Then we get outdoors in full kit and wonder why we can't stay in the drops for long.
I bought my safety clear and sunglasses when I only rode hybrids and just needed the bifocal setup for reading my phone, etc., without needing to carry reading glasses. But on my road bikes the clear overhead frame support slightly obscures my vision from the drops, so I crane my neck up more and get tired more quickly.
I have another set of frameless sunglasses, but the upper part of the lenses mashes against my forehead and quickly fogs with perspiration and skin oil. Same problem as the other glasses.
Occasionally I'll cheat and omit the glasses for some routes, but only if there's no traffic and it's not bug weather. I've been struck by pebbles and larger debris too often in traffic so I don't go bare-eyed unless the road is pretty much all mine. There are a couple of training routes where I can ride 5 mile loops and see maybe one or two cars, so occasionally I'll cheat on those roads.
And I have one cycling cap, but don't wear it on the bike. There's no visor position that's comfortable to me. I use bandanas for sweat control. No interference with my helmet fit or peripheral vision.