With something like squarespace or wix (or even WP hosted), you're putting more power in the hands of the centralizing host, which is both limiting but also can reduce security issues.
The "sheer breadth of the ecosystem" in self-hosted WP is also where so many of the problems come in (compatibility between products, security issues, etc).
I'd argue it really is worse in the WP scene vs Drupal, partially because of the 'ease' of the code for newbs to get started. There's no culture of automated testing in the WP community at large, but some other platforms at least allow for that. There are people who write clean and well-tested WP products, but they're likely a minority, if you're looking at the ocean of stuff released over the last 5-10 years in the WP space.