I watched a video where someone tried out boob shaped armour- the problem wasn't the deflecting IIRC, it was that the armour itself came to something like a point against the chest so any impact HURT.

I don't think it's an issue that they'll be deflected towards the heart, no arrow, spear, sword tip or hammer's spike will pierce the heart through a decent breastplate.

The best I've ever seen accomplished is denting or a couple centimeters of penetration. That wouldn't be likely to happen in the center of a breasts' plate as armorers as far as I understand always tend to leave areas like that fairly thick.

I might be underappreciating the metallurgy that happens in a crease like that, and the amount of stress it'll be put under with so many strikes directed to a center crease.

The main issue I see is if something is deflected up towards the neck. There's usually a "V"-shaped ridge to prevent that, even in this illustration. But I'm thinking that a couple domes offer a lot of variation and maybe opportunities for something sharp to bounce past the neck guard and slide right into the gap. You should likely have a thick collar and some mail there if your armor includes a mail coif (hood), but those are a bit less protective than the plate. Good mail is still pretty hard to get through, though. But getting a few centimeters of a spear through is enough, and that's not atypical from the tests I've seen. I'm guessing even just the impact can crush your throat and mess you up, even if the mail manages to withstand the thrust.