As far as golds I regret spending on, Untouchable, Raw Nerv and Parasite Weave have to be the ones I regret. Raw Nerv was back when I first started out. She was my 2nd gold ever (first being UV). For a while I didn't know about the existence of the SGM discord, the forums or the SGM subreddit). I stopped using her when I started getting more golds besides her, AF and UV. I thought her haste and enrage were godly, but when I actually realized the circumstances of gaining these buffs, I knew she sucked. Parasite Weave used to be good, but I was just unable to get to 1-10% at all for a long time (keep in mind, this was back when LINE still owned this game). I knew she was extremely good, so when I finally got her I was happy. But this was close to the time when she got nerfed hard, so investing in her ended up being a waste. Now she just gains dust and I sold all of her moves afterwards since I stopped using her. Untouchable's one I'll probably get cut slack for. She's not the best for offense, but I maxed her out in an attempt to get better on the defensive. But honestly (and you can probably cut slack on me for this), she's overrated for defense. Her health isn't THAT good and her SA is easy to get around. Yes, her SA1 could save her from fatal hits. But honestly, 15% is not a chance I'd count on. Plus, there are plenty of ways to work around it. She's honestly never made a huge impact when put on my defense teams. Plus I care more about offense than defense. So I decided that i'd use her to diamond my UV, which I obviously don't regret.
In regards to silvers evolved to gold, Robocopy, No Egrets and Scarlet Viper are ones I regret. Robocopy's kinda obvious so yea. No Egrets can be good, yes, but with a Primed and Sheltered competing for Parasoul moves and sp, I eventually abandoned No Egrets. Not to mention lockdown is somewhat weak at the moment. Its good, but in most circumstances, its probably not going to be enough since No Egrets is somewhat lacking in damage. Scarlet Viper's fine, but considering I have a Bloodbath at 7k FS now (making her usable for me now, I don't see much reason to use her. I'll probably get around to her eventually, but for now, she's just gathering dust.