The point being that they are not at all user friendly. Look at this case alone…the OP is considering the replacement of the whole unit, primarily because of difficulty of him being able to effect any kind of modification of his current device. So having to pay for a new monitor and accept a lower quality of component in order to stay within budget.
Having separate components that are easy to service, whilst not creating a bigger footprint, would be a better choice. Especially when access to shops is limited. No doubt OP could fit more RAM to an NUC/Mini PC. Whereas I wouldn’t even suggest that he tried with his AIO without seeing which model is involved.