That's the downside.  

It has helped immensely.I am going to be making lots of assumptions here...Assuming the turnkey company is top notch, has boots on the ground, knowledgeable with that respective area, has integrity, good tenants, and the area chosen has a healthy or growing market...is the only downside lower ROI and Monthly income than if I did most of the tasks myself or parts of it myself? 

The downside is assessments.

Also, are there any unforeseen downsides to using this method?

If you do T/C, the 9% T/Cs are really hard to get (at least in OR) since they're competitive.3) If you buy a property, find out when the income-restriction goes away since the property will be worth more - All other things being equal4) On the plus side, you have no problem getting tenants even if they're a bit harder on properties than your average tenant.5) On the down side, if income-restricted, you're at the prey of HUD for rents you can charge. 

Is there any benefit or downside to doing both at the same time? 

The down side is that each one needs its own corporate tax return, but the upside is that the cost to file was pretty much identical with the series LLC or close enough that it didn't matter. 

The only downside is you have to recapture depreciation and it is not shielded by sec 121.But if you meet other requirements, you can take advantage of it.

What's the down side, if my returns are better than the class A/B neighborhoods?

If there is no such requirement, is there any downside to this plan?