tldr -- I think that even with very complex and delicate mods OR large quantities of blueprints you are afraid of possibly losing OR both, your chances of any problems migrating to v0.18 are very remote. PM me if you wanna chat by text or on Discord, I might even be able to walk you through the process of literally guaranteeing no loss of savefile or blueprint progress, which I've become rather proficient at for reasons that I hint at in my longer message below.
lovely_santa, I am still running on 0.17 since I don't want to risk breaking my save.
I'm aware that my comment doesn't answer your question, and I get annoyed when I pose a question and receive a seemingly unhelpful reply like the one I'm about to give. So, just understand that I'm genuine and not trying to be sarcastic.
I can report that v0.18.16 is very reliable, and I'm not aware of any savefile breakage, despite the fact that I manage at least four different mod-loadouts (I float between several saves for various reasons, each one with its own special set of mods, and use the sync-to-save function to change which mods are loading.) I can report from personal experience that the mod "Configurable quick start" (that's an exact quote including case sensitive, there are a few other mods with false-positive name searches) can run fine in v0.17, and also runs fine in v0.18, but throws a very obscure error ONLY WHEN loading in v0.18 a savefile that was created in v0.17, which I reported here: https://mods.factorio.com/mod/quickstart/discussion/5e3f55fc215b8f000cfc8859
Note that within that thread on this website, there is also a link to where that mod author posted on the forum to the devs, which has not been responded to yet and probably never will (because of how obscure the use-case is.) Note that as long as the "Configurable quick start" mod is used in v0.18 gameplay on a map that was originally created in v0.18 then there's no problem. For my own purposes, since that mod is only useful to me at game start and I don't use it during gameplay once I spawn into a new map for the first time, for my own purposes I just leave the mod disabled (but kept updated on my system) and only manually enable it for initial game creation and that's it.
I understand that this is a long winded tangent, but I think it's relevant here to describe the fact that that's the only save-breakage I've personally experienced in v0.18, and only the third save-break ever in my four years of playing Factorio -- with the most recent one before this the break in v0.17 when they changed the science recipes around, and that one is reversible with mods anyway.
Conclusion: I sincerely think that there's little-if-any danger of breaking your save. As a bonus, I volunteer to help you troubleshoot your own scenario if you would appreciate the help in migrating to v0.18, as I also occasionally bounce between Factorio versions and I have gotten rather proficient at preventing savefile or blueprint data losses. In fact, just today, I reverted first to v0.14, then to v0.16, then back to v0.18 when I was done, all to use a blueprint string that I found in a forum post dating from 2016 for a pet project of mine. PM me and maybe we can talk on discord.
