They are demons.

Smart, loving, sweet dogs that care for and would do anything for their herd. But you have to train them well and keep them mentally and physically engaged. You’ll have to work hard to socialize them and make sure they behave when folks come over or even walk past your house.

My wife is a pushover and inconsistent. (Not her fault, she’s naturally gentle and that’s her brand of ADHD.) Training our little gremlin was a real challenge. Picked up potty training from our other dog immediately, but he was a pandemic puppy and socializing wasn’t the best. Fine with strangers, but is anxious and barks too much. Barks at folks walking past the house, etc. Hate him, but I also love him. Little beastie gave me tinnitus.
Ours is a mixed breed - half Australian cattle dog, half American pit bull terrier. We do okay with managing his energy levels by taking him for a half mile walk 5 days a week and sending him to daycare 2 days a week. He could probably do a longer walk, but our other dog cannot (She’s old and has arthritis), and he won’t walk without her. He’s fairly anxious - weirdly, getting a cat seemed to help with his anxiety. They wrestle a few times a day, but don’t like, hang out or snuggle, but he just seems calmer and more confident. Ours is also intensely jealous and basically wants to be the center of attention for whoever his favorite person is at the time - which does change on a daily basis.
The recommendation I’ve seen online, but have not tried, is that if you’re going to get one heeler, get two, so they can wear each other out. It tracks with my experience, unless you have a lot of land and a decent amount of time. Heelers are working dogs, and even though they make great pets, their energy and need for engagement and exertion need to be considered.