As a monk main....

Solo instances aren't so much an issue for damage coming at you. Additionally, most instances have an NPC healer that helps keep you alive, so as far as being squishier is concerned, this isn't really that much of an issue, except that maybe you have to be a little more aware of damage mechanics than you'd have to be on a tank/healer role as failing might mean you take more damage than the healer is going to realistically top you back up from.

BUT the the big problem for monks espeically, but for all melees. The solo instance bosses still have standard hit rings meaning that unless your solo instance also pairs you with an NPC tank, hitting positionals on attacks is, for the most part, impossible. Sure you have True North to help if you're going to burst out a lot of damage, but the fights last longer than your position nullifying effects last meaning for the most part you'll be hitting for lower damage than is possible in normal dungeon or raid scenarios.

To answer your question more directly, samurai you will notice this disparity, not because your GCD skills will hit for less damage, as that's not how samurai positionals work, but you'll notice you don't gain as much kenki, which will still impact things like what OGC moves you have available to you at any given time and will mess with the flow of the rotation you've come to expect.

And on this point.... Square really should be making these hit rings for areas you're likely or required to be solo omnipositional. So solo instances for sure as well as open world mobs.