To be fair, any WP hosting company is going to run into issues with non-standard installs, and they're not going to be able to accommodate any setup. If you have a customised WP install, it might not work on this sort of provider, they're optimising for speed, not dealing with, and they probably don't want the headache of trying to support any plugins or modifications that you might bring to WP.
It would make sense for them to offer a dev plan with every production plan though, and let you upload your site, and serve it to a limited no of people as a testing ground. Given their 'Power Tools for Power Users' stuff on their website, they really should consider at least offering staging servers for testing.
I guess at present they're just not a good setup for developers trying to modify WP, and are instead trying to capture the market for hardened WP hosting for mostly vanilla installs.
"and are instead trying to capture the market for hardened WP hosting for mostly vanilla installs."
I don't know of any large WP installs that would need the benefit of outsourced WP hosting/experts that use 'vanilla installs'. They may exist, but I can't imagine they're a large portion of the market. Perhaps my scope of experience is too narrow in this respect?