bore restriction from poorly soldered joints
You're right, simply down to bad soldering practices. That's the main reason to only flux the pipe and not the fitting when soldering. The main culprit for solder in the pipe is when flux is applied to the fitting and not/as well as the pipe. When the flux filled fitting is pushed on, the pipe squeezes the flux inside the pipe and then the solder is drawn in when heating. When it's only on the pipe the flux is pushed to the outside of the pipe and wiped off before soldering so the solder is only drawn into the gap between the pipe and fitting.
I will always prefer to solder instead of push fit or compression where possible (of course compression's been around almost as long as copper soldering has) as I prefer a system that can last 50 years or more. I still add to and alter soldered copper water systems that have been in leak free service since the late 50's/60's and earlier in some cases typically found in the old pre-war Glasgow tenements where the lead was replaced. Never seen copper pipe so thick in all my life, that and the old 3 piece riveted hot water cylinders - great down the scrappy
Unfortunately I couldn't say that for any of the legacy plastic/push fit/compression system I've ever come across, especially the systems put in during the copper shortages 70's>80's - Brown plastic push fit, black/gey alkthene (solvent and compression), s/s pipe w/compression are all failing now after 30 odd years. Even some of the 'new builds' from the 2000/2010's where their plastic pipe and fittings are now starting to fail now, guess the proof is in the pudding.