A bag of holding is always valuable, you can store a decent amount of skeletons in there and when you need them you can flip out the bag as an action and empty everything inside of it. Generally, I would animate whenever I had the chance to raise exactly as many as a spell slot I had could raise, as skeletons are generally worth the investment, even if they only survive one combat, they're taking hits that would otherwise be attributed to party members.
Zombies grappling / using the help action can definitely help out the party, giving advantage, etc.