In my experience, I found the teachers who dished out detentions, were the ones where the pupils were disruptive in the class. Probably regarded as the ‘strictest’.
Yet there were a few teachers, who had control of the class. Not due to threats of detentions or punishment, but due to having the respect of the pupils, relating to pupils etc. Children were motivated to behave well, do homework etc.
You cannot generalise - even when you are a teacher in the same school, I do not know what goes on behind a classroom door and neither do you, as a parent.
I give detentions according to my school's policy and my behaviour and attainment is good (I would never say perfect - no class group, teacher, student, parent is perfect 100% of the time) but last academic year (before my maternity leave) I had the best homework hand-in record in two year groups and pretty damn good in all the others. All of my students met or exceeded their targets (first time that has ever happened and probably down to a lot of the guess work in setting targets currently happening but that's another thread).
My students know where they stand, with 2, 4, 6, 8...missed homeworks (odd numbers are always just a written warning in my school) - there is a clear sanction progression. Along with, at the back of my booklets, reward stamps (5 means a merit) for students achieving their personal best, whole class hand in and class team points etc. Students have, in the past, moaned to me when they get a new teacher and the setting of homework becomes inconsistent again because the teacher wants to be 'liked'.
Generally, students like to know where they stand with praise and sanctions - either one being inconsistent is a recipe for disaster. Apologies if this was not your intention but your post comes across as generalising that strict teachers (you know strict teachers can also be fair?) have awful classes and fun/happy/easygoing teachers have great classes. Absolutely not the case across the board and I do get the impression that you direct experience with your DC has lead to very sweeping statements from you that don't necessarily help the OP.
OP, I fully appreciate that this is already being dealt with by you but it does look like some mistakes have happened between the SENCO and the teacher in question for what it's worth (that shouldn't have) and I really hope that this doesn't happen again for your DD and that any future sanctions are set with your DD's SEND needs considered. Detentions don't have to be same day or even after school all of the time and I hope that you and your DD get a good outcome to this.