I got a tube of them for her when she was just a puppy, and she bounced around the house gleefully with her first ball, until it popped a hole in it so loudly that she dropped it and leaped away, then picked it up and ran to me. She shoved that ball in my lap and backed away, looking me in the eye and mouthing the air, very clearly meaning for me to fix it.
She's become less sensitive about their welfare.
Because she was going through them so fast, my very generous coworker let me in on a secret: she'd played tennis the entire summer before, and they opened a fresh can of balls every game. She had a whole bunch of tennis balls, practically new, that Elka could avail herself of. She brought the bag to me at work, and I took it home that day. I got in the door and Elka stared at me expectantly, so I did what any reasonable person would do and dumped the entire bag of balls on the floor. She ran away, but then came back to investigate.
Then she picked one up and gave it a proper Elka welcome, the back-breaking shake.
Tennis balls are our premiere backyard play toy, where there isn't time to stop and chew them like a wad of Big League Chew, because if Elka pauses for too long on the retrieve or settles down to do just that, always have more to throw! They aren't so great in the snow, though. Now that the thaw has fully happened, there are probably about ten of them out there.
However, I've also become aware that tennis balls such that people use to play tennis aren't actually good for dogs to chew on. Apparently there's a layer of grit in them that wears their teeth down. So, trying to be a good responsible dow owner (I really don't like terms like "dog mom". I might as well get that out there early), I've since been on the search for suitable replacements, that she can chew on without actively ruining her teeth.
However, I don't think these manufacturers have dogs themselves.
I used the above ball a single time, at the end of a walk. I'd run out of treats, and brought the ball to tap into Elka's play drive as a reward for walking well on the leash. Her walking was fantastic. As you can see, the ball did not fare well.