I think most developers who "dread" working with WordPress are those who are comfortable working beyond WordPress itself, which could mean other frameworks and/or languages.

I've worked with WordPress even before the 1.0 days, and what I don't like about WordPress now is that there are too many WP-specific knowledge a new dev has to learn to implement simple things. Before, you can simply play with the theme's files and use plain PHP/HTML to get exactly what you want; now you need to understand how to enqueue scripts and styles, learn WP_Query, etc.

WordPress used to be a simple tool to get a basic blog or CMS running, now it demands a bit more from the developer. To some people, the "re-learning WP-specific things" part is what they simply don't care for.