That point about weird layouts is so true, in my experience. When you're dealing with older homes, the key is to have a variety of transition fittings on hand because, as nathan_brown59 alluded to, integrating with existing fixtures can be a headache. I've found that using push-fit connectors for temporary fixes while you plan a more permanent PEX route saves a lot of time, but your mileage may vary depending on the wall access. It's messy, no doubt, but if you can map out the old plumbing runs before you start, you can minimize the damage to walls during retrofits. Honestly, the ease of repair often comes down to how prepared you are for those unexpected material mismatches.

Extending @ryanjohnson, silence makes brain's faulty timestamps deafeningly obvious.