Scarabs compete with Wraiths, Destroyers and Tomb blades though - Tomb Blades are an excellent fast option that would be really hard for me to pass up, Wraiths are of course an obvious scalpel unit, and Destroyers, while not my style, are certainly a good pinning unit (not in terms of the pinning rule, but in terms of forcing enemy units to keep to cover or get vaporized). Scarabs also tend to have a more predictable point of entry and can be dealt with before lines close. It may be of greater benefit for a canny player to eliminate all the scarabs they can for the sake of removing the Spyder's ability to replace them.
Flayed Ones don't need to be fielded in such quantites to be a spearhead assault element when utilizing outflank. They also have far less competition in their own FOC slot - I can see people wanting to field multiples of anything in the FA section of a Necron army, but the elite section seems to be much more selective. So if you want to stuff flayed ones into a list it's not going to disrupt it much regardless of the type of army you're bringing. Scarabs on the other hand might require more of a sacrifice, and also immediately make you want to drop in tomb spyders (though of course these are hard to pass over even without scarabs).
At any rate, the two units make sense in the CC-oriented army working side by side. If I end up sticking with my old force - which would be far cheaper than buying what I need to make a high-speed warrior focused army - I'll probably be using 2 units of flayed ones and 1 unit of scarabs. I just need something to force the enemy into the middle of the table so I know I can reach them. Still not sure what the third elite choice would end up being; either Lychguard or Praetorians, most likely.