Ramp = They can just unweld it
Hole in wall = six pieces gone = harder for them to easily fix
Ramp + hole = multiple points of entry
On certain maps more than one point of entrance is a great thing, allows you to come from all sides and overthrow the enemy very quicky.
I just dislike ramps because its just too easy for the enemy to unweld, if I can do it to theirs fairly quickly ( sometimes behind my own walls ) then they could probably do the same. I find it complicated to fix a hole in a wall if there are no spare pieces around ( usually the case since everyone uses up all the pieces....usually

)
On a map like skywalk, it can be complex to weld through all that since its such a small map and yes maybe impractical. But on larger maps, especially resevoir or city 6 panels is nothing if no one is paying attention to that specific wall or I got backup to keep them off me.
The smaller the map, the more complex it is for the engineers because the walls are easier to keep tabs on and the sparks are the dead give away.
Best way to keep engineer's off your walls is to make sure all your t'mates know that sparks = bad ( You'd be surprised how many idiots I have come across that think sparks only come from repairing a wall...lmao )
To sum it up, ramps are effective if you can keep their engineers away from it (IE. Keep rushing to prevent them from getting out of their walls ).
Making one hole is hard to fix, making MULTIPLE holes is nearly impossible to fix if you steal the pieces and they have none left at the moment.
Just depends on the engineer