EC, or external cups, the manufacturer controls the cup, the race and the bearing so it's fairly easy to get a good seal.
Similarly, ZS, or zero stack is the same. The manufacturer controls the cup, the race and the bearings and you can get a pretty good seal.
IS however, seems to be a different story. The "cup" is the responsibility of the frame manufacturer but the bearing and the race are by the headset manufacturer. There always seems to be a gap between the frame and forks. Has anyone found a way of sealing this gap properly?
Used to be able to get a small strip of thin neoprene with a snick of velcro at each end, that wrapped around a lower headset (I can't for the life of me remember the name right now) but the one I had worked well for many years until the velcro eventually came to the end of its days. It did need to be removed for a day or so after washing the bike to let any moisture back out, otherwise rusty bearings were a possibility. May still be for sale; whether there's a 1.5" version is another question.
Edit - it's just come back to me, think it was called HeadSox
I tend to marine grease any bearings now.
Pro/Con's
Old threadded headsets need the whole thing replacing if the cups get pitted - pita if you don't have tools. Cheap unless you have the old threadded headsets with cartridge bearings - no-one makes the bearings - Shimano 1" and 1 1/8 I'm looking at you Shimano (although they are strippable and serviceable with non caged balls if needed).
Cartridge bearings (external and integrated), then £20-£30 on bearings, usually just the lower needs doing.
I consider them service items now - lower bearings need doing more often. I'll strip and refill with marine grease - resists the water.
I find water can get down from above and settle at the bottom of the headset, so grease it all.
Anything 'covering' the bottom race will just trap dirt etc, and wreck the paint - hense why replacing a lower bearing is easierst (if a pita with cables etc.)
I used to run a bit of inner tube over the bottom race many moons ago. I think it just kept the crap in.
This thread is more a grumble that more as more headsets seem to be going IS but it seems to be in least well sealed.
I found headsox and similar to be terrible. They just trapped dirt and moisture. Now i just buy Cane Creak headsets and forget about them. They last for every in all weather.
Every FSA headset I've every had has quickly rusted in the bottom cup/bearings
Fitted a Hope headset about 3 years back to replace whatever crap came from the factory and lasted 1 winter. Not a Hope fanboy who demands everything Hope but it's done the job and not needed touching.
About 25 years ago I had a WTB Greaseguard headset. Both upper and lower headsets had a grease point on them. Use a grease gun to push fresh grease in and the old manky stuff comes out. Seemed like a great idea. Don't know if you can still get them.