If you take a look at their quarterly production numbers, things don't look so rosy- they jumped from ~25000/quarter in Q4 2017 to 80k/quarter in 2018, but have not really moved the needle that much since then- this past quarter was 96k (96,155 if you want to get pedantic). Its fine growth for sure, but certainly not in a hockey stick matter- it makes me wonder where the disconnect is- how and when are they booking the revenue, and can this be expected to continue to jump if production isn't growing accordingly.
All the while, they are still losing money. This is not just about cash flow, this is about actual earnings. You have to look into where the disconnect there is as well to make a bull case.
I agree that the growth is likely to slow down somewhat in 2019. On a years view they produced about 253.000 cars in 2018 and will likely land at 360.000 in 2019. Nevertheless it is still 30% Growth YoY.
I would argue that slow down is mainly due to the nature of the production ramp, which goes exponential in the beginning but is now starting to flatten out. As it flattens though margins should continue to get better. If they get additional factories like the one in China it should reinvigorate the growth rate.
For me the bull story goes something like: they sunk a lot of money building out their very first mass producing production line, making a lot of mistakes on the way. Although they have some quality and service and margin issues people still keep buying their cars, which is the most important. Fixing the margin and other secondary issues is just a matter of time and continuous optimization.
So the most important of Musks hypotheses that people like buying electric cars if they are better in every important category is pretty much proven by now.
The second, that they are able to produce them at a profit is still to be proven, but economies of scale play a big role in that and they are first beginning to really leverage those.
It isn't just about cash flow, but both cash flow and earnings matter. Their cashflow has improved to the best state that it has been in since the company was founded.
Their other issues are a combination of scale and fundamental business issues with selling cars. Other manufacturers have profitable dealer networks selling their cars. Tesla has a money losing sales and service network. Some of this is due to their relatively small scale, but some of the problems are deeper than that, unfortunately.
All the while, they are still losing money. This is not just about cash flow, this is about actual earnings. You have to look into where the disconnect there is as well to make a bull case.