Well its a room in a tower, and presumably maaaaaagic
Don't go anywhere in Mallus, its too dangerous.
Regardless land vs sea is just far too selective on a case by case basis to make generalities.
I think the TLDR is if you're going anywhere off the beaten tracks, you need to have something going for you. Even if its just a big, extremely well-armed convoy, its still gotta be more (probably a lot more) than normal. This applies to land or sea.
Its case by case sensitive. Such a sweeping generalisation generally isn't helpful, but as far as I can see, overland in the Old World...generally pretty safe along as you are not in an area like Sylvania.
Same for sea travel cept for the sea of claws.
Rivers are better than both, they're faster and generally have fewer gribbles in them save for places like Sylvania again. Even then its still a better option generally hence things like the stir river patrol.
Leave those areas which are "civilised" though things instantly become more dangerous, until you reach areas like the badlands. Here sea or land travel you're kinda ****ed and what is "safer" matters much less than what is practical.
In my opinion, land travel through the bad lands is probably the safer way to reach cathay or Ind for example. Ships are all or nothing and can't be reliably fixed en route. Caravans can and if you lose them then like the viceroy your odds or survival are much better than bobbing around holding onto wreckage.
Of course this will likely change as ships themselves become more advanced. If they can be made faster, mount more fire power etc. I can see people like Marianberg or Tilia putting together trade armadas. Just massive concentrations of force centred around cargo ships that sail to the east and bring back vast hauls.
Just not really practical at the moment.