|Smurf is grim for smurf knows the odds|
The main issue is try as we might, it is hard to conjure up memory of any playing a "balance" list, kick arse and lives to tell the day when the said player stand atop of his/her enemies' smoking corpses. The main problem of being "balance" seems to be 2 folds.
1. You can do everything, but good at nothing.
2. Good at nothing is in essence meaning you hold no edge over your enemy
Meanwhile looking from the other side it will be like:
1. Edge list will focus on what they are good at and play accordingly
2. Against a "balance" list, primary threats are easily identified and resources are invested swiftly to neutralize them.
For example suppose you played an mix/max list of landraiders and terminators, automatically you will scan for where the serious armour/terminators threats are in the opposing army. That would mean say against another imperial army you would just want to look at where the meltas and lascanons before you reached the line and after that it will be on the look out for CC death stars and plasma guns/demolisher canon after you breached the line. Everything is clear cut, you can obvious like say ignore that landspeeder which is armed with two heavy bolters for the moment.
Where as being "balance" say you have 50% of units in your army being able to do decent AV job and 50% of them don't, it will actually be like fighting with 50% of your army the whole time.
The same seems to apply to the basic law of operation in reality, imagine going to a job interview and telling your interviewer you can do everything but aint particularly good at anything -- it will be like asking for a general worker's job.
So thought of the day? Get an edge for your army!