Yes, altitude is not accurate but that's more the nature of trying to determine altitude using pressure. I doubt you will find any watches that can consistently give accurate altitude readings.
Measuring altitude using pressure is not like measuring depth using pressure. Water doesn't compress and so the pressure is directly proportional to the mass of the volume of water above. All you need to know is the density of the water and the pressure and you know the depth. The density is determined by the salinity of the water. Note that rarely do you know the exact salinity of the water you are diving in so even depth measurements will be slightly off but as all the tissue calculations are based on pressure it is of little consequence.
Air pressure is a whole 'nother can of worms. Just like water pressure in bodies of water, air pressure is determined by the mass of the volume of air above. The higher above sea level, the the less air there is above. The complication is that air compresses so its density is not consistent. Additionally, weather conditions (high pressure systems, low pressure systems, etc.) also influence the pressure reading. How can a watch account for these other factors?
Here's a scenario I use to explain the problems:
You are sitting on your back veranda sipping a beer. It’s a beautiful day. The sun is out, it is warm and there is a gentle breeze.
You calibrate the altitude on your watch using GPS.
You finish your beer and fall asleep.
You awake 90 minutes later and the weather has turned. It is now cold and very windy. It has started to rain. A storm has rolled in.
The barometric pressure has dropped by 50mbar since you calibrated the altitude on your watch. A drop of 50mbar is equivalent to an increase in altitude of more than100m!
Your watch detects this change in barometric pressure. What does it do? Does it increase the altitude? If it does it will be incorrect (even though the pressure reading was correct) as your altitude has not changed.
Does it maintain the same altitude? Why would it do that? How does it know your altitude hasn't changed?
This is a relatively simple scenario. What does the watch do if you were out and about when the storm rolled in and your altitude was actually changing? How can the watch maintain accurate altitude?
Here is some interesting information:
How to Choose an Altimeter Watch | OutdoorGearLab
Also check out the altimeter accuracy in this review of 10 watches. These watches are all a lot newer than the Mk1 and even they have accuracy issues.
The Best Altimeter Watches of 2019
