I feel like you've been caught out by some exaggerations, the only thing that's any harder to work on for the MK18 versus any other variant is the removal of the front rail due to how Marui opted to design the barrel nut/rail interfacing. TLDR you need a specific tool if you want to remove the rail with any degree of ease, BavTac make one, though there are others (it's an RS tool if memory serves but the name escapes me). You need to remove the rail and outer barrel to gain access to the hop unit, and given that swapping out the hop nub is the most commonly recommended change needing a specific tool to be able to do so is a bit of a bastard.
That aside they're identical to the other variants of the MWS, bar the MTR which I believe has an ambi safety. The MWS is extremely easy to work on, with the caveat that 'easy' is a subjective term and everything is hard until you know what you're doing. Though the process of learning what to do is fairly straight forward with the MWS. If it's the MK18 look you're after it'll generally cost you more to gather the bits to turn a different model into one than it would to just get the Marui one, at least assuming you want it to be relatively 'clone correct'.