For example when you drag and drop toolbar items onto your form it automatically gives them a boilerplate name. You can rename these but you tend to find devs would forget so after a while just looking at the code you started to get Label1.Text = "blah". I seen that in just about every project and one I remember done by a Chinese dev with poor English he left all the controls with the boilerplate names and so it was impossible to reason about what was happening.
Componentisation was also a problem in that you tended to have all your code connected directly to the form. So if you had a large form that done complex stuff it wasn't unusual to have a multi thousand line long form file.
Separation of concerns was a problem in that again the code was attached to a form so you found UI code mixed in with proper business logic. This becomes very messy over time and tends to create very large event handlers the more a project grows.
It was basically impossible to unit test which makes refactoring very risky. On large projects what generally happens then is that no one is willing to refactor anything as they don't really know all the scenarios. Over time this leads to code rot.
It only ran on windows, while I'm sure there are ways to make it run under linux the main support was for windows and most orgs stuck to that. This ties in with the fact that you had to install the thing on your P.C. businesses hated this and still do. Things like people uninstalling it or problems locally with permissions and suchlike were rampant. Similarly updating was a pain on large projects you have to have something central install and do the update.
Scalability was problematic on large projects especially since the forms app would often directly connect to the database. That meant the more user = more connections which on large project meant problems. The "solution" was some service or api which the app talked to but at that point it's no longer vb6 doing the heavy lifting and it's just become a UI.
Small little tools or prototype apps was were vb6 shined brightly.