30mm bottom bracket and 30mm spindle axle compatibility, should it go in by force?

The fit off the axle in the bearings needs to have no play between the race and axle. The axles are designed to be tighter than a 'slip' fit, but not a full 'interference' fit. This means the axle will go through the bearing inner race with a little force. Often, hand force is not enough.

You can use a soft mallet to carefully tap the crank through the bearings. It's a good idea to protect the drive side crank arm with a few layers or rag.

See this Park Tool Repair Help page: https://www.parktool.com/blog/repair-help/crank-removal-and-installation-two-piece-compression-slotted

There is no difference in the fit expected based on road vs mountain.

I thought I would be able to provide tolerance numbers from the original BB30 technical documents put out by Cannondale. They used to be at www.bb30standard.com, which is defunct now but can be found on Wayback. However, turns out it seems to have only ever had shell dimensions. The spec seems to have left the specifics of spindle and bearing tolerances to manufacturers.

Generally speaking, it's a pretty tight slip fit, same as you're used to. I understand that you've measured with calipers and gotten 30mm exactly for both. With higher precision tools you could see more of the story, i.e. if something is out of round or if the parts are actually requiring a press fit or what. (Note that among commonly encountered vernier calipers, some have .05mm precision and some .02mm, which could be enough to make the difference in this case, which it usually doesn't with anything bike.)

I've encountered similar situations where it took some force to get it all together, but if it was a press fit I would be unwilling to do that unless Rotor told me it was how it was supposed to be, which I don't think it is.

Hammering in the spindle is not a good idea because side loading the bearings like that can damage them. It's the peak loads from hammering that can do it, and knowing how much is too much is a guessing game. Gentle taps are probably okay, especially if it works, but I still don't love it. I believe the notion is that radial bearings are supposed to be able to handle 10% of their allowable radial loads in the lateral direction. Usually when it's tight I try to walk the spindle into the first bearing by wiggling it up and down with force behind it. I deal with a lot of BB30 and do not find myself having to hammer cranks in basically ever.

Update: I think you all were right. I have not installed the bottom bracket yet, I want to have the rest of the parts of the bike before (I am building it); But I used a bank-screw to hold one side of the bottom bracket and used a mallet to slide it in. It went through, and after the first section it could slide. It seems that the first part of the axle is wider by a margin my caliper cannot measure, but enought to make it tough to get in.

I must say that it was just as tough to get it out of the BB, but I guess that's not a big issue, as long as it does what it is supposed to do.

Thanks all for your help!