Are animal companions ever balanced?

Are animal companions ever balanced?

Only when played by the DM.

They are always balanced.
For martials they are crutch not to lag so far behind.
For casters they are just cosmetic addition because the caster is already powerful even without them and the little extra makes no difference.

players always seem inclined to protect or avenge animal companions/avatars regardless of their practical function - death of an npc is whatever but go to the ends of the earth to save your dog or something

Depends on the animal.

Smaller companions have ungodly stealth and agility bonuses and having something that can fly or see in the dark is always useful.

Dogs are incredibly underpowered imo, mostly because people assume the average mook is far more competent in handling them than is reasonable. A pair should be more than enough to take down a human opponent by themselves.
>and they should get massive bonuses against skellingtons

Animal companions are balanced when the DM realizes they are just well trained animals, that can't do everything a familiar can. While your pet wolf IS a bad ass, in the end it's still an animal.

I remember once a rogue in my party managed to tame an amusingly small bear (only 3 ft high or so) and brought it with him as his adventuring buddy. Unbeknowst to us, he was under a reducing spell with a duration of 2 days. Turns out he was normally 6 ft tall... On four legs.

He was a big guy

For u...rsine creatures, yes.

Obligatory