Working opposite muscles! Good or bad?

[SO19 Soldier]

I used to only do workouts like this:
Back and Bi's
Chest and Tri's
Shoulders, Legs and Abs. I done it like this as was informed it's no good to work opposite muscles in the same workout as you're working them against each other. For example, chest and back or bi's and tri's.

I was then told by a good friend who gains well and has been doing it for about 10 years that its rubbish and you can do as much of everything you want. But he's also said my first injectable course would be good on deca and sust which seems like a big no from everyone here so thought I'd find out.

I've been doing more than just 2 areas for a good few months now and seems all good as I can do much more in a day and to me I find it much better.

So what does everyone else think? The back and bi, chest and tri way? Or do as much as you can way? Also is there any negatives from doing opposite muscles like bi and tri in the same workout?