The lock-in is that it's a big pain in the arse to use anything but their IDE.

Most vendor lock-in isn't "it's impossible to do the thing" but "it's hard enough to do the thing any other way, so this is effectively the only practical way to do it"

But the vendor in this case is Atmel and the hard way has existed before Arduino was created. The contribution of Arduino was that they made the simplified path - it doesn't make sense to accuse them of lock in for that.

> The lock-in is that it's a big pain in the arse to use anything but their IDE.

How so? It uses bog standard avr-gcc and avrdude. There is nothing stopping you from using those yourself.

What's hard about it?

It's also possible to import the Arduino libs/headers and build against them with a little bit of Make.

I put together a simple setup to skip the arduino ide on an AVR design, but still be able to use their serial.println and other utilities. You can use it side by side with manual register masks for enabling IO.