I'm at Rank 6 right now and have to say I'm loving the missions - it has some issues (which I'll post more about) but overall really nice.
I really like the feel you get from the missions leading up to the snarbolax and the fact that you are given a cobalt set makes you really feel that you have graduated to becoming a knight and are ready to go into the clockworks.
However I'm feeling that at around Rank 4 there needs to be an introduction to the arcade and here's why. The following missions are all about getting you ready for different damage types, but if I were on the production server I would not have nearly enough cr to buy any new equipment to take advantage of this training. At this point I wouldn't have even gotten my current equipment heated up. Since mission kind of suck you in you just want to keep going - a lot of new players I feel will just keep going in their cobalt set - completely missing the point of the type damage training.
It's also not clear that you can repeat missions at this point - so players will completely miss that they can keep beating the Snarbolax to heat up their gear, get more crowns and get more tokens. (side note: boss tokens are kinda glossed over in the token tutorial - it should be made more clear to new players what boss tokens can give them)
I think a arcade tutorial at this point will help a lot. It will give new players a chance to do some grinding (which will be needed when they don't have free energy and cheap gear like we do on the testing server). It will also help slow them down from trying to beat all the missions at once without being properly equipped.
It also looks like your going to be phasing out the old objectives (because they do not fit with the mission system at all). So you're are going to need an arcade tutorial at some point.
The mission system seems to mostly be a repackaging of current gameplay, and has so far about completely discarded the old packaging, while I feel that the old packaging should play a much bigger role.
Especially because you can't just hop in halfway through a mission; you have to join at the very beginning, while from the arcade you can hop in about anywhere. This makes missions seem a bit more solo oriented than the arcade. This conflicts with SK's team oriented nature.
I would be fine with this if the arcade was introduced much earlier, before rank 4 even, to allow for easier playing as a team. That, or adjust missions to be tolerant of joining partway through, to make it easier for groups to form.