Hi Wombat
The TTL standard gives a lot of scope to manufacturers to create confusion. A logical "1" should be half line voltage, or greater, a logical "0" is 1 volt or less. Some receivers put out line voltage and ground, some half line and ground. If the device being driven, servo, ESC, switcher or whatever requires the full RX line voltage as a logical "1", then problems can occur if used with an RX that only puts out a 2.5 volt "1".
Having done a component count on a few likely circuits I came to the conclusion that its actually cheaper, as well as more compact, to buy ready-made. The only time I don't now is when I do a small one, when I butcher a servo. This gives an ESC and a motor in one go. The limits are a very narrow deadband, and a max of 6 volts supply and a motor that doesn't draw more than 500mA.