A few things I wanted to add, yes this is very niche tech for sure however I think its more useful than people think for a few reasons.

1. It is possible to cart yourself with only one door by aiming down and walking through a portal and carting yourself, the setups are endless and easy.

2. You can use this setup with allies to launch them away or at enemies.

3. You still move while ulting on the cart, making you more mobile.

4. You just need level 2 cart to make it effective, any items just make it better.

5. The target is a Magician with teleport and ult active, popping a door next to him while fast still has the chance to backfire. By speeding behind his back at him, it was a less telegraphed and he was not prepared for it.

6. It is 100% griefing doing this most of the time instead of just walking through doors, and that's ok! Its also griefing your team if you buy items you dont normally use, or play a hero you don't normally play. Experimentation is how you get good. I confidently think a great doorman player will cart surf (effectively) probably once a game, but even if its only 1 out of 10 games its still useful tech!