There is no noticeable signal loss from using cables to connect separate VCR and DVD recorders, and no improvement from the interior connection in DVD/VCR combo recorders. If anything, the combo recorders can be more subject to interference due to the exposed, unshielded ribbon "bridges" connecting their VCR sections to their DVD sections. Also, Pioneer only marketed one or two "low-end" DVD recorders in the US, mainly the model 231 which *was* pretty lame. Most better-grade Pioneers with hard drives are very good (540, 640, 450, 550), as is the Phillips 3576. So don't base your decision entirely on that one bad experience: most any current DVD recorder is capable of great recordings.
I'd say in my experience the results from combo recorder vs separates ranges from roughly equal to much better using separates. Much depends on the specific combo recorder and how it implements its internal components. If the combo has a very good quality VCR section, the limiting factor will be tracking concerns and variability of source tape quality. If your tapes were recorded primarily with the same VCR and primarily off the air, a combo recorder can give excellent results. (Though some early combos ironically had encoders that were really unsuitable for VHS dubbing- the worst of all worlds.)
If, like me, you have a collection accumulated over the course of many years using many different (and long-defunct) vcrs, made from assorted cable systems, satellite, OTA, and 2nd-3rd-4th generation copies, you will probably have an easier time getting good results from separates. The same borderline recording will play back differently on every VCR you try it in, so I keep four on hand (Panasonic AG1970, Mitsubishi DVHS, JVC DRMV5 combo DVD/VHS recorder, and an old Quasar). All of these feed a Pioneer 640 DVD recorder with 160Gb HDD.
DigaDo gives excellent info re his use of combo recorders for a big project, but note he was using OLDER, "deluxe" Panasonic combo recorders which had convenience features that are missing from most other combo decks, new or used. The typical combo deck has "dumb" automated dubbing features that make it MORE difficult to use than separates, and remembering to switch back and forth between its tape signal and DVD input monitor can be a pain. If you are very dextrous and can keep track of the kinda goofy operational ticks of a combo unit, you might love it. My own preference is for separates, I find it much easier to use the two remotes and two machines.
Short version, it all depends on your source tapes and work habits. Good tapes are conducive to using a combo, spotty tapes are better served by separates. The "quality" difference is tied the VCR's playing ability, most DVD recorders (or the DVD section of combos) will record in excellent quality.