Orion but...should it be better?

Good to know about the iOptron... so far I feel like I've been lucky with it. Last summer my first outing with it yielded the photo below, so I was more than slightly stoked.

What are the practical limits of time on the iOptron? Focal length dependent of course, but is it unreasonable to talk about 8 or 10 minute exposures at 400mm or so?

I know about the star rotation...we're the ones moving hehe! I just use "the stars move" colloquially, like "sunset" etc...
I think (if I'm phrasing this correctly) that my question was about tracking error in relation to the z-axis? In other words, that a hypothetical perfect barn door tracker is capable
of longer exposures than a hypothetical perfect iOptron Skytracker? Assuming they're both perfectly aligned and all...?

IMAGE LINK: https://flic.kr/p/xu8K​3W

I never could get anything more than 2 minutes with my 150-600mm lens on my iOptron even with a dead on alignment, after that star trailing really became apparent. I don't think 8-10 minutes is possible, even 5 minutes with my 50mm and 24mm I started seeing trailing.