I think the worry is particulary with turbo cars it will clog the lines to the turbo which would be pretty bad for your turbo. I remember when I started doing the oil changes in all cars in my family many years ago, particulary the bighorn it took a while for the dirtyness to go away (used to services every 10000km+ i suspect knowing my old man and car maintenence) but after some regular 5000km oil and filter changes it should clean up and then not have the need for a flush, least thats what i think/how ive done it. My truck has 340,000 km on it engine untouched. For the sake of $40 for 5L of caltex delo 400 and $30 or whatever for a filter I think its worth doing both every 5000km, if you do the oil not the filter then its definitely going to go black faster as the oil filter holds lots and obviously it wont filter as well. My truck usually dosent burn enough to need topping up between changes, and this is the only oil I have found which dosent burn away! Maybe its because its semi-synth, but valvoline and other oils just burnt away. If the past owner of the truck was really poor with maintenance then I guess you may have to pay for that. After reading a thread on here by 'electrifying' and how his sump mesh filter was clogged with sludge I really want to pull off the oil pan thing and check mine out.
Actually i also remember some theory that old engines where the pistons were effectively being nicely sealed by stuff that could be flushed away by these engine flushes, effectively making your engine rubbish.
I really wanted to do an engine flush to try stop the lifter tap in my VR4 (its not too bad compared to some you hear) but held off the urge since its not a problem and dont want to risk screwing something else up. That stuff you buy to stop lifter tap is useless and dosen't work.
As for injectors ever since low sulfur diesel came out ive used a few squirts of moreys diesel additive with every tank and havent had to get anything rebuilt due to diesel leaks yet, the stuff is meant to be good for seals, clean injectors and increase cetane rating so all sounds good to me
Ill have to have a party if my truck makes it to 400,000