Way more people competing for way fewer homes.

As long as you're looking at three or fewer you're fine no matter what.Great plan!

Also, remember that the reason the prices are so high is that these areas are incredibly desirable, which means you will likely have a different caliber of tenant and property than in some of the "cheaper" markets which could mean fewer challenges as a landlord.

Interest rates are low, and there are way more people in Denver than ten years ago and they're competing for way fewer homes than there have ever been. 

If you're working with fewer than 10 open lots, they won't be a good option. 

One cost seg company told me the process is not difficult or time consuming for a single family or small multifamily.

I might even say, the fewer rentals you have and the newer you are, the more conservative you should be with your numbers, especially going in...with the rehab numbers if it needs much fix up. 

Both of these options are costly and time consuming.  

@Will FraserIn my year+ of feverishly consuming podcasts, books, and forum posts on REI, I don't ever remember someone bringing up the possibility of a retainer.

If you have the funds to tie up the property and obtain the entitlements which can be very costly and time consuming that is something of value.