One huge disadvantage to private 1 off sellers is that they are aware of the countless Investors trying to break into the market, and they are insisting that their properties are worth market value when in actuality they are not.

Website builders have another significant disadvantage.

Another way to think about the issue might be that if EVERY agent pays for Zillow ads, then any agent NOT paying for Zillow ads is at a disadvantage.

This can bring advantages and disadvantages of course.  

With that being said, generally speaking from those of you who have experienced both or had to decide between one or the other:As an investor, compare and contrast the benefits/disadvantages if you had the same amount of resources to purchase several multiplexes with fewer units each vs one larger multiplex with many yet nearly the equivalent number of units (i.e. you can buy four 4-plexes or buy a 16 unit property with the same money)?  

The disadvantages is that I don't do showings when there are short term tenants so it can be inconvenient for potential buyers (but I doubt its ever cost me a sale) and its a ton of work.

If so what are advantages or disadvantages of this approach?

There are also TICs, which have significant disadvantages of decentralized management, and DSTs, which evolved as the successor to TICs to solve the decentralized management issue but they have limitations of their own. 

I personally put a high value on having tenants I KNOW wont screw me over.What are the disadvantages?

Not really, but even if it was it's good information to gain perspective into why people prefer one over the other, if there was no relevance to the issue than I'd say there is no relevance to purchase 5/1 units which we all know would be a huge problem.I like to understand the pros and cons of the advantages and disadvantages of properties to make choices in what I purchase.