As long as the majority of the parents and students in my class are happy, I don't worry about complaints. There will always be parents or students who have something to complain about, so all you can do is take that complaint on board, and judge it on its merits.
I had a new girl join my class one time. After about a few months I was told the parents had complained. They told me the mother had complained that she comes home and doesn't speak English. That she's been learning English for a few months and nothing has improved.
Now, I'm the kind of teacher who likes to manage his class. You can't teach kids if you don't manage them at the beginning and show them how they're supposed to learn. This girl joined late from another school. It took a few weeks to get her adjusted, as she didn't know how to listen, sit still, not talk when I'm talking, etc. I teach kids what I expect of them from the beginning. And once you've mastered that, the kids will know how to conduct themselves well in class. And that means you've laid the foundations for a solid learning environment.
The worst kind of complainers are the ones who do it because they're insecure and need to assert themselves. Hell, I've even had farang bosses do this to me. I can only ascertain that they didn't like the fact that I knew how to teach already when I joined and didn't need them micro managing me or constantly asking for their help.
I had a pair of farang bosses actually call a meeting with me as they were very concerned about my teaching style. One of them had never actually seen me teach and was just taking the word of his partner in crime, who'd only seen me teach once, and even said it was a good lesson. I remember how they had clearly rehearsed the whole thing before. And there was me, pretending that I was taking them seriously, nodding and agreeing while they referred to one of my students as a "ghost student". Apparently that's a quiet student who doesn't participate. Only problem was they named a student who was the complete opposite of that.
As soon as they said it, I was so relieved (neither of them had any idea who was who or what they were like). It was just confirmation that these two clowns felt so threatened by a teacher who knew how to do his job, that they made up a load of BS so they could assert themselves to fill those huge insecurities in their lives. It was an utter embarrassment. Oh, and they were so worried about my teaching style that they never came to observe after.
So, lots of complaints will be valid. I think anyone who doesn't take themselves too seriously can decipher what's legit or not. Sometimes, unfortunately, you will get insecure idiots feeling the need to try and control you or tell you what to do just because they can. I think these people have really shown themselves during covid. It's just a shame there are weak and insecure idiots like this in education, too.