As a preface, I've modded a bunch of games over the last decade, and used reflection in every one that had code modding to begin with (Minecraft, Rimworld, Stardew Valley, Terraria…)
In each and every one, reflection is *necessary* if you want to do anything beyond simple (or fancy, really) items. Mods like IndustrialCraft, Create, Calamity, the Vanilla Expanded series, hell, Forge itself are *impossible* to make using only basic coding methods. And, while it can be kind of considered an API issue, the truth is, API never has and will never have everything a modder needs. Even games with deep modding integration and communication, like Rimworld (and it's genuinely built around the fact that people will mod it) don't have an API wide enough.
You claim that reflection makes mods unstable, but it doesn't. Badly used reflection can do it, but my literal decades of experience tell that players are quick to recognize and quietly ignore mods with subpar coding, and they rarely survive more than a version or two. What it really seems to be, from my modder perspective, is a simple "my code! mine! don't touch it!" impulse, which is a silly impulse to have in a game revolving around a modding community this much.