They are but they also very different Character Fantasy-wise. Plus they have their own gameplay quirks that are not very compatible with the Pure Muscly Fighter role.
If you go for spellblade you need to invest in some other Magic classes, and propably look/craft equipment that boost Magick Attack alongside regular Attack
If you go for Assassin you should at least consider "In the Shadow" because otherwise you're ignoring one of the Major bonuses of that class, and also you'll need to take off your Plate Armor and at that point you're no longer a tank. You can function as one if you then pick Ninja and start stacking Mag Def and Evasion, but character fantasy. That character would be some sort of Ronin who keeps easily dodging and parrying enemy attacks before retaliating with his sword, which is also cool, but a different kind of cool compared to an armored juggernaut that swings a big stick and shruggs off everything enemies throw at them
Also technically Assasin is not strictly melee class. It works fine on Archers too