My humble opinion, it is simply more to memorize. Not only do you have to memorize the mnemonic you have to memorize when to use it, how to use it, when to omit something in it or add something in.
TOMATOFLAMESFLAPS is useful in only understanding there is too much crap to commit to memory, look it up in the reg. as an example
Briefings are meant to be brief. Talk about funkiness on an approach in cruise, get the basics covered again down low.
TOMATOFLAMESFLAPS is ridiculous. Mnemonics shouldn't be longer than 7 or 8 letters AT MOST. There's science out there suggesting that's about the limit of working memory for humans. I use lot's of mnemonics to jog my memory in the airplane about what I should be doing when. I use the same mnemonic D-WARTS for takeoff with a slight variation on what each thing means.
D = Departure Procedure
W = Weather (any weather out there to be concerned about)
A = Abnormals during takeoff and what to do
R = Runway Conditions just like above
T = Terrain (just like above)
S = Speeds (just like above)
Works great for me, and when I get busy in a single pilot airplane, if I lose my mental place because of a radio call, or some other situation, I can come back to D-WARTS and go through it quick in my head without it being a big deal.
Similarly, for every airplane I've flown in the last 5 or 6 years, I've had a mnemonic that takes a few seconds to run, but covers all the "Don't Die or Embarrass Yourself" items that I can do as I taxi out. In the King Air 90 it's:
CIGARTAB
C = Controls (Free and Correct, lock removed, etc)
I = Instruments (Up and Erect, Heading's Good, Altimeter is set right)
G = Gas (Plenty of fuel, fuel panel set up right)
A = Avionics (Good Squawk, Good Altitude, GPS Set, FD/AP set)
R = Radios (frequencies are good, things make sense, I've got the right frequency in the flip flop)
T = Trims (Three Set)
A = Annunciators (Extinguished or Considered)
B = Bleeds
Before when I was flying the PC12 it was "GLIFTY," the Navajo was a "FIGART," the 1900 was CIGARTAB (pretty much like the king air), the only airplane I didn't have one for was the Caravan - and mainly because that was essentially the most brainless airplane I've ever flown and there weren't many things to miss.
I do that after I've done my checklist items because in my experience - when you're by yourself, a mis-timed radio call from ground or other distraction can break your train of thought when it comes to your checklist usage, and single pilot it's easy to get out of sequence and not notice it. As such, I run my checklist, then back it up with a mnemonic every time, and I've found that the amount of things I missed decreased substantially. Mnemonics aren't a cure for all that ails us in aviation, but they are useful tools for keeping you focused on target in the single pilot world.