There is one component to playing fretless you have to be aware of regardless of whether you have a lined or unlined fingerboard. You have to have a very good ear.
It isn't any harder physically. If you have been playing fretted for a while, you fingers already know where they should go.
It is a different sound and can be fun to play.
As to those who advise against it, I wonder if they have considered the fact that bass players from the 16th to the 20th century had no fretted basses to learn on.
The common technique for beginners is tape on the fingerboard, but with side dots or a lined board, it doesn't matter. Personally, I see lined fingerboards as training wheels.
They are great for some applications and don't work at all for others. It is just another tool in your kit.
One thing I can't do is sing and play fretless at the same time. I saw a video of Jack Bruce playing a fretless Warwick and singing lead on Crossroads. I am not Jack Bruce.