At least with those you never had to worry about a reboot unless it was a kernel change, everything else could be restarted without down time. But the complexity goes up and that's what many don't like. It also requires more of a team effort amongst developers at least from my point of view because each part of the OS becomes it's own entity. This can both help and hinder security since there is more talk going on between the parts of the OS leaving space for injection of malicious code. A well implemented OS of this type would fly, absolutely fly on modern hardware due to how it would be stored in memory, but that also means a very intelligent and streamlined memory manager would be needed in the base kernel. Imagine only needing a kernel footprint in memory of like 5MB or so everything else is a module of the OS and can be swapped out as needed to keep physical memory available for applications. In many servers this may be a moot point because often they only have a few simple tasks but in a desktop this would be a head turner. I mean you could have Netbooks with a gig of ram that would fly simply because memory would always be available for a running app and not filled with the monolithic OS kernel. I could also see this fitting well with modern virtualized deployments since it would allow a greater density due to better resource allocation. Hell this kinda modularized OS would be a perfect fit for a hypervizor since it could bring the footprint down to near nothing and lower the over head for running guests. Hmmm, good food for thought here.
Just my $0.02
