The problem is that there always will be some OP things that get better when spammed. Either massed ranks of weak crap, or if you will not let anything wound anything the though things, because half the enemy weapons will be useless. There used to be rules to prevent spamming with the force oginization chart, but with all the detachments that has gone out of the window. The problem is they wanted to make all old armies possible. And thus you automatically allow spam armies. They should have made a simple mechanic of 1 troop allowing another unit, 2 troops allowing one of each unit. And max 2 troops for each other type of unit. It would not make spamming impossible but would limit the worst spam. And possibly they should also say no more then half your units can be (inside) vehicles like they did with deep strike and some people thought initially when they read every other unit can have a transport vehicle
(Yes I know we would hate that too, so probably the wrong forum to post this on) But the general idea is to force more balanced stuff. Also some points should be adjusted a little bit. A psyker for 20 points is very good, giving them 10 wounds makes it insane. having a 7 point swarm is good, giving it 4 wounds makes it better also giving it 8 attacks is to much.
Allow spam lists but let it be from upgradable force organization charts that costs points. You get the standard 1 Hq 2 troops and 1 of each for free. You want anything beyond that: you pay the points for it.