Well, the relationship between rfactor and GTR2 has always been odd; rfactor came later but has fewer features, OTOH some things (such as assigning AI setups) are much easier, but the menu system and UI in general is a kludge. For reasons I cannot quite determine rfactor seems a much better platform for open wheel series (which is all I have installed in it).
I generally do not use mod installers at all, for any game, having been bitten too often with overwritten files, unwanted cfg edits, etc.
I've always stated ease of creating mods is essential to the popularity of any sim; it seems many "modern" devs are moving away from this concept, taking the attitude of Micro$oft - "here is my program, use it the way I designed it, not the way you want to". (GPL has been the exception here; notoriously difficult to mod has meant the few mods available are very high quality. But its original design was merely to represent one group of cars at one group of tracks for one season, mods were never considered.)
And oddball proprietary file formats, encrypted files, and other such atrocities are a dealbreaker for me. Not too long ago I downloaded car sounds for a newer sim hoping to adapt them to GTR2 (having found none for GTR2 that I liked, and not yet wanting the trouble of creating my own from scratch). What was extracted was a bank file, something I'd not seen in years (and used for music quotes then); spent an hour downloading and trying several recommended programs til i found one that successfully extracted the contents of this ...only to end up with a conglomeration of oddly named wav files and a few files of unknown type, complete gibberish, deleted the entire mess.