I know there were a few times when I began using the Propeller, I ran into trouble when I had two cogs which could change the same variable. I rarely have this trouble anymore. If two cogs really need to change the same variable then there is likely a flaw in the structure of the program. For example,
my Mecanum Wheeled Rover 5 can be controlled from a RC remote (usually used with a helicopter) or it can be controlled over the USB port to test the various functions while on the bench. These two input devices don't directly change any of the control variables but instead the main control loop monitors these inputs and adjust the control variables appropriately.
As you suggested, I find if I'm careful about only having one cog change the value of a variable, programming in parallel becomes easier. If one needs to make sure a single cog has exclusive access to some resource, a lock could be used. There are 8 locks in the Propeller which can be used to block access to a piece of memory or other resource until the cog using the resource releases the lock. (I occasionally use locks to share an UART or I2C bus between multiple cogs.)
You're right this type of programming can be challenging but I think being able to write code which will run concurrently is a big reason I enjoy programming the Propeller so much.
IMO, it's a great microcontroller for robotics since it allows one to control multiple devices while simultaneously monitoring multiple sensors. Want to control 32 servos?
No problem. Here are
22 servos doing something a little more interesting.
The Propeller is very Adafruit friendly. Both 3.3V and 5V devices work great with the Propeller (5V inputs just need a 3.3K resistor to limit the current to 500uA). It handles NeoPixels with one hand tied behind its back. I ought to (and probably will) make a YouTube playlist titled "Spinning Adafruit" or "Propelling Adafruit" to show off all the Adafruit products I've used with the Propeller (BTW the FONA and the Propeller are a great match).
Did I mention I'm a fan of the chip? I'm also a fan of Adafruit. (UPS tells me my latest Adafruit order is out for delivery. Woo hoo!)