Risk of leakage when changing open-vented to sealed-system boiler
While it's difficult to quantify the risk, based on my experience, I believe it's quite real.

I went ahead with the sealed system, and I've experienced since then a couple of leaking radiators (e.g., at the venting valve). I think the system might also be losing water from somewhere else that I haven't realised because the system pressure went too low on a few occasions, and I had to top it up with water manually.

So, I'm not saying it will happen, but if you're facing a similar choice, I think you should weigh in this risk as well.