Horses for courses.

The best courier I personally deal with (DHL) has a similar rating via online review sites, but I've never had a problem with them myself, in several years of using them. People are motivated to trash businesses they have dealt with (like you're doing here) if they had a bad experience, but - typically - are significantly less motivated to leave glowing reviews, even for years of great service (I've never left a review for the aforementioned courier, though I have recommended them to others in appropriate circumstances).

Most people already know they are brokers for Fastway / Aramex, and Couriers Please, and the thing about that is it makes Sendle only as good as the designated courier service that's used in specific areas, quite a few of which are franchises, and service quality varies greatly. Which is why any time Sendle are mentioned in various seller groups I visit, there's plenty of people saying they use them successfully with no issues at all, and of course some who say they're awful and to not use them. 

I won't use them,  I don't particularly like the way they operate,  and because consistency is efficiency as far as I'm concerned (i.e. my buyers tend to know what to expect with AP, and I don't have to exclude any addresses like PO boxes, and / or use multiple carriers / pricing models.

As for "do you care enough about your buyers to refrain from using Sendle or is it a case of hey, they'll pick up my parcel and they’re cheap – who cares?"   this is a false dichotomy.

If I put my most cynical hat on, let's say a seller's only concern is their own interest, which means they have to decide if it's worthwhile for them to use a specific carrier to get their products from them to their customer. I don't think any seller in their right mind would continue using a carrier if even 20% of their sales resulted in complaints, let alone 90-odd %. Even if they didn't care about their customers, they'd definitely care about the time-suck from sorting out problems, bad feedback ratings etc that these things often result in.