Ascending horizontally requires the diver to be
neutrally buoyant and have the capability to fine-tune variation between positive-neutral-negative buoyancy using breath control. In turn, this requires precision weighting.
I wouldn't dispute that horizontal ascents aren't harder - but they do foster and promote a very high degree of buoyancy control. Whilst initial development is harder, the long term benefits are much more substantial.
Conversely, using vertical ascent techniques ultimately enables the novice diver to compensate for poor buoyancy by using the legs. That compensation reduces the need for perfected weighting, breath control and precision control of the BCD. That makes things easier... but also serves to retard the divers overall 'feel' for buoyancy and ability to perfect their technique and weighting.
If a diver has perfect control of their buoyancy, they can ascend in ANY position.
PADI teaches feet first descents, exhale and then sink down etc... This is a retarded way to dive for an experienced diver, or one who needs to pull down an anchor line in current or one who needs to descend quickly or vertically in a current with no visual reference.... HOWEVER, it is fine for TEACHING people how to dive when they are first learning.
I taught my kids to freedive first, so they were never taught to do feet first descent.. just flip over and kick down...(the way REAL divers do)..
You seem to be implying that different amounts of ballast would be necessary for a vertical versus horizontal ascent... that makes no sense to me at all?
However, I STRONGLY believe that performing ascents (especially without a rope or visual reference) can be difficult for new divers. They are more likely to be in control, and have better visual orientation and are therefore actually SAFER, if they FIRST learn to ascend in a vertical orientation, while striving to maintain an ever so slight negative buoyancy.
Trying to ascend face down, spread out, DOES require the diver to be ascending in a buoyant condition (for at least portions of the ascent) or they would NOT ever ascend.. they would just hang there...
I have no problem with experienced divers choosing to ascend "like a sky diver", but it is not the way to learn the skill.. in my opinion.
Buoyancy control is really learning to BALANCE an unstable equilibrium. It can be demonstrated, but it is never really taught.. the student needs to LEARN to balance themselves... just like you can't TEACH a kid to ride a bike,, they eventually just have to learn to balance it.