Opinions will vary hugely. If you only hear good feedback about a school and can't find not even a single person who says anything bad about it, chances are the people are not being honest (like those who live in Crapford-on-shite, endure a 90-minute commute, but tell you the train journey is no big deal and they couldn't be happier) and/or you haven't done enough due diligence.
The most important thing is understanding how relevant the experience of other people is to you. Is their child more academic than yours? Less academic? They loved the sports facilities but yours doesn't care, or viceversa? They wanted to do music and drama but yours wants to do further maths, or viceversa?
"Strict" and "academic" mean different things to different people, so always try to understand how relevant other people's experience is to your situation.
The point on how subjective it all is becomes crucial if you have special needs and an EHCP plan. A school which may be great for a child without special needs may not work for yours.
Extremes are always dangerous: schools with too little discipline and schools with extremely draconian rules can both wreck mental health, although in different ways and affecting different children differently.
Look at the policies online. Is there anything that sounds off? Ashcroft in SW London is very clear that bikes are not allowed and will get you a detention; Michaela in Wembley is very clear that not SLANTing (look it up) is not allowed. Other schools (e.g. Harris Wimbledon have a reputation for being strict but the head is adamant that she doesn't agree with those methods and would never want to work in a school like that. Only you can know what works for your child.
Speak to the parents. But beware of selection bias. Kids who cannot bear the discipline at Ashcroft either don't choose it or leave in the first year, so current parents are more likely to like that.
Once you have narrowed down your choice to 2-3 schools, try to visit them more than once, if at all feasible. We have had very different vibes from a few schools when we visited the second vs the first time.
A worrying trend I have noticed is headteachers caring less, especially about special needs, once their school improves its Ofsted rating and/or becomes more desirable locally. Why try harder and worry about the "difficult" children if the school is popular anyway, many seem to be thinking.