Too much LM can have negative effects, you only need a tiny amount and it HAS to be level with no beads in the paste, if it beads up, it can crack the die. Unlike TIM, you don't want to just put a load on and let it spread with the weight of the IHS as it can fall out and hit other parts under the IHS causing potential damage.
If you don't remove the glue properly, it will cause an uneven surface and weird temps, I had this when I reglued mine, there was a tiny bit in the corner I missed whilst delidding which resulted in two cores being ~60 and the other two ~80. I'v only delidded once but learnt a lot. You only need like half a grain of rice when it comes to Liquid Metal and application is important here, if it's not smooth / level, it could crack the die.
I think that's what he means by too much LM and fail glue cleaning.