A mechanical swing is a really great gadget to have in your house. It gives mommy time to quickly have a shower or make dinner. As long as you don't overuse and let it take the place of tummy and floor time, it is good stimulation for baby.
As for bouncers, I assume you are looking into this for later on when baby is older. As with any carrier, walking ring or bouncer, the key is that it MUST support the baby's whole pelvis, baby must be in a sitting position not have legs hanging straight down. And this is hard to find because most products on the market only supports the crotch area. I didn't pay enough attention to this when I had my first baby, and he developed a pelvic problem because of my ignorance. So from my own personal experience, I would beg caution when using products that make baby dangle from the crotch. Baby should be sitting, and the support coming almost up to his / her knees under the legs.