Stability and readability. While ActiveX stuff is fairly stable, it still has odd issues from time to time. With a proper language, as long as you don't change something, it will keep working without problem. If what you create is reasonably complex, trying to find or modify anything will be a massive pain. Since you will place the trades yourself, it's not as bad but I wouldn't let it run unsupervised. I used both the DDE and ActiveX versions. DDE was garbage, ActiveX was usable but "flimsy".
Thanks for the reply. Are there any alternative tools you'd suggest for what I'm aiming to do? I have programming experience but I'm intimidated by the idea of creating a UI myself - that's why easy to use cells and VBA was attractive to me. Can I ask when you last used the DDE and ActiveX versions? Am I right in thinking Excel only works with DDE and to use ActiveX I'd have to go about setting up my own app in C# or python?
IB includes an ActiveX Excel example with the API installation but you can create your own as well. I used Excel ActiveX, some 6 or 7 years ago last. Creating an GUI is not that hard, I have limited experience with GUIs myself. I started with Tk in perl but now I use Python and Qt exclusively - it's quite intuitive. I was able to create a very crude, basic backtester with a GUI in about 3-4 months. I'd recommend that route, Qt is excellent.