And it's easily replaceable in case of failure. Of all the design shortcuts this one isn't bad.

Everything cracks under pressure, I'm not exactly sure what the above commenter is getting at. If the sub was steel the walls would be thinner. With titanium the walls are thicker. Without knowing the dimensions of the material we can't know whether it was built to high enough standards.

I mean, anything will crack under pressure. The biggest issue I see is uneven compression of the two materials coupled with different fatigue behaviors. I'd feel a lot safer if the whole submarine was titanium, honestly. Barring that, a couple inches of solid steel would be just as comforting.

I would be worried about both. Joining two very different materials that need to deal with crazy pressures seems like a really bad idea.