Logging in to machines to update configs doesn't scale well. You often end up building your own framework/api "only" to edit the config file.
Or paying $2500/instance/year for nginx plus.
Or paying $2500/instance/year for nginx plus.
Maybe nginx isn't better than haproxy for many purposes. But updating configuration isn't the reason :)
If you're at a scale where logging into each server from your control node 'hurts', then you're still going to hurt with all the config other than nginx anyway.