Self Leveling Compound for Floating LVP vs No Leveler and Glue Down LVP
It really depends :

How out of flat are your floors?  Have you taken a 10' straight edge around the room to get the flooring and found the largest difference?

How picky are you, if the floor has some bounce/flex when walking on it is that a deal breaker?  The constraints are there to give you a high quality installation.


Glue down won't work if the floor is so out of level that the clicking mechanism doesn't line up or if you do manage to click it you could end up with voids that cause bouncing and eventual failure of the product.
Some techniques I use:
Consider furniture placement.  If you have furniture covering the area of the floor that is less perfect it doesn't matter as much as you won't notice the bounce and that section of floor won't be subject to continual stress.
Fill low areas with glued down high density underlay
Sand down high spots ( depends if you have a localized high spot ).
Plan the direction of your boards depending on the direction of the slope.  Typically you'll find that the subfloor is flat going in the direction of the joists but across the joists it tends to go up or down depending on how the joists were installed and how much crown they had.  If you put the boards parallel with the joists then the click locks will likely work better than if you put them the other way and the click/lock is trying to stay together over what to the floor looks like a void.
Really I think an 1/8 is more than my perfection requires.  Anything 1/4&quot; or greater should be fixed ( based on the straight edge ) I use a 1/4&quot; horse shoe shim and try to fit it under the straight edge as I move it around the room.
If you have dips you can use SLC.  Mixing it up properly isn't rocket science - though a scale and remembering that 1L of water = 1kg helps when mixing up less than a bag and figuring out mixing ratios.  You should have screeds, trowels and choose an SLC that accepts feathering.  Most require a primer to prevent the plywood from absorbing all the moisture.  I found myself doing a couple lifts the first time and it worked out ok.  I hired out some of the work after doing a few smaller section myself and realizing how hard it is on your back.
I had a room where the end dropped by 1&quot; so I raised it 1/2 with plywood and did SLC to bring it up the rest of the way.
SLC can and does crack.  That shouldn't be an issue if you have underlay and the subfloor is flat enough that anything that could crack doesn't move around.  Cracks are really fine the worst are pieces that come loose and then rub together as you walk on them.  Underlay is fairly soft and any pieces that do some loose are likely to embed in the underlay.  If the subfloor is flat and the LVP is installed flat and has underlay I don't see how pieces of the SLC are going to move around.
If you want perfect results you flood the entire area with SLC such that it covers every square inch of the area and you move it around as it does not self level.  You are really better paying someone to do that level of finishing.