Is it just accent or that people make too many grammatical mistakes? Or to put it another way, if you had to choose between two: clear pronunciation with a lot of grammatical mistakes vs a strong accent but flawless grammar. Which one is harder to understand for a native speaker?

I have had a few Indian professors who tend to have perfect grammar but strong accents, and some Chinese professors with poor grammar but better accents. I find the latter much easier. It's easy to correct simple grammatical errors in my head, but if you don't know what words are being said, it doesn't matter if grammar is perfect.

Thanks, that's something I thought, but wasn't sure, because as a Finn it's hard to get enough samples of the equivalent situation: someone speaking my native language without accent or without grammatical errors.

I've heard from some Finns that they have never heard of a foreigner speak Finnish at a level comparable to a native.

Contrarily, I have heard several Europeans speak American English almost flawlessly.

It's not that bad that Patrick has the accent, but he talks really fast. I struggled to listen to his talk at Hacker School. I have talked to him solo in person and understood him fairly well; maybe it was just the auditorium.

Do you mean Startup School? I don't recall him talking at Hacker School.

Yes, I meant Startup School.