May I just ask how VB.NET code isn’t maintainable?

TL;DR: Lots of old code written by VB6/VBA programmers in the wild. It's way more holistic than C-hacker-ish C# code.

Speaking from a bunch of legacy projects, you will likely encounter:

• Option Strict Off activated, so your type checking and IDE tools are half broken now.
• Global imports from anywhere to anywhere, proper namespacing is unheard of.
• Object everywhere, because Variant.
• On Error Resume Next everywhere, so you switch off and on exceptions while debugging.
• ByRef everywhere, because VB Classic is call by reference first.
• All declarations on top of a function, drastically increasing their scope, because, you guessed it, this is the only way in old VB.
• Code bases riddled with poor reimplementations of string methods using Mid. Actually, that's a bonus, it's very satisfying replacing those with one call to a string member.
• Multiple assignments to function name variable instead of return, making code harder to follow.

I'm currently maintaining a multi million line VB.NET code base, the foundations of which were hastily laid down by young and inexperienced devs realizing a business opportunity in the early 2000s. Lots of these out there in the enterprise world from what I hear and I think this is where there the language gets its reputation from. Sure, at its best it's just C# with words in place of curly braces, but that's only the case with well disciplined programmers (and even then, why not just use C#?). Option Strict is, well, just an option, and even the infamous On Error Resume Next is still usable in VB.NET to this day afaik. A lot more room for shooting yourself (or the next person reading your code) in the foot if you don't know what to look out for.

I was mostly thinking about PHP with that comment. Which has some serious issues with how modules from other files are included and general structure. It's possible to write well organized PHP projects but it takes discipline, it doesn't happen organically, and its really hard to fix once the project has grown significantly.

I haven't used VB since VB.NET 2003 so I hesitate to speak on that directly. Professionally I work across multiple OS's and architectures so all .NET languages are kinda no-go's. That's where C++ really shines.