I use the adjustable screw on connectors and they work fine a little thread lock and they are not loosening up at all,I would recomend using the more permanent metal retainer rather than the rubber one that can be removed, it will loosen up over time for critical areas there handy for multi task servo situations..
if you file a flat in the pushrod like you have stated there a perfectly fine connector for all uses not just for throttle.
when I have a dedicated servo the I will use the Dubro Quick connect the one that slides over the "L" bend and snaps on the pushrod there reliable and simple,a good rule whenever possable keep it simple,the good old Z bend is also a viable and reliable method of connecting pushrods to servos.
here is the inside of my latest P-47 the pull/pull with the rudder pushrod using a EZ connector,my forward servos throttle and retract servos with EZ connectors and the one not hooked up will have a Quick connect for the elevator,as well as for the ailerons and flaps.
what I don't use is soldered on connectors, when done right there fine but a bad solder joint can fail, when I use a snap on type I will use the screw on end nylon connector.
all my warbirds,the 4 I currently have are 4st powered, its about comfort level if you feel the connector your using is solid and can pass a basic field test moving the control surface with out slipping, then there you go, if it looks like it may be fragile with potential to fail then its a no go.