Apparently the only relatively safe strategy is to rely exclusively on items on the DLS. In effect that's what I do, since built-in items frequently can't be renamed which makes them impossible to find in a very large base of installed assets. This is particularly true of items which are labeled payware, and which I automatically disable. A waste of disk space as far as I'm concerned. (This of course doesn't apply to dependencies such as engine sounds or bogies which are not usually chosen directly by the route designer, though even these can snarl things up after an upgrade.)

Unfortunately, there have been cases of items formerly available on the DLS which in later versions became payware, with the result that even attempting to install an older version from a previous installation of Trainz is instantly obsoleted upon submitting. There seems to be no rhyme or reason at all to how this issue is handled. Sadly, this has become yet another reason for not upgrading, which I would think would be the opposite of what N3V management intended.