As a 40 kg Mastiff mum, I wanna share my experience with flexi leashes. I believe it would be useful for those who have recently adopted/got a puppy.
I moved to Berlin from Istanbul with my girl, Mona, almost 5 years ago and since we used to live in a house with a garden before, she has never walked properly on a leash. In Germany, if it's not a fenced dog area, it's not allowed to unleash your pup in a park, therefore, I happened to get a flexi leash in order to make my baby girl move and sniff more freely.
Flexis are terrible. They encourage bad lead walking as your dog will never learn boundaries. It will make loose lead walking and walking in a 'heel' position way harder in the future. You feel like you're in control - but actually, you really are not with an extendible leash. He/she grows up pulling the leash and being ahead of you the whole time, which makes it harder for you to control him/her in the long run.
Also, once I mistakenly dropped the leash, so there was a plastic brick reeling and bouncing itself towards Mona, making her run off further.
More and more significantly, she was constantly pulling against the flexi due to the constant tension. I switched to a regular leash, and after a couple of days, she was not pulling almost at all. If you have a young pup, 'not pulling' learning would take longer than my experience with Mona because she was already 4 years old.
All in all, briefly I highly do not recommend flexi leashes.
If you have additional questions about this topic, you can reach me via email@example.com.
Thanks for your time,
Sending kisses and hugs,