Versus unmodded, the only thing that might be missing are some graphics options. I'm not entirely sure about that.
Versus my heavily modded ME 1 / 2 /3 games, the only thing I missed was being able to remove shared cooldowns and stop shields from blocking biotic powers, both of which were major retcons in ME2 and ME3 that I won't accept. Within 24 hours, there was already a Coalesced editor available that let me fix those things. The other major bits of modding were for graphics and the stupid ME3 ending. The graphics no longer need mods (in my opinion), and I won't be getting to the ME3 ending any time soon. At the rate mods are appearing, that probably will be fixed by the time I get there.
So in short, no, I don't feel like I'm missing anything important from my old modded games.
Regarding bugs, I haven't encountered any game breaking ones yet, but I don't discount the claims of people who say that they have. All I can say is that for me, it's been stable. Frame rates have occasionally been disappointing. Not bad, but also not always smooth -- not sure what's going on with that.
Only you can make a proper decision as to whether you should keep it or refund it. I'm happy with it, and definitely getting my money's worth.
