I would argue it is possible to believe both that Tesla is a great company and that Romney was generally right in that many of the green investments have indeed been losers (Solyndra, A123, Beacon Power, Abound Solar, ...).
Lest we forget, Romney is/was a pretty great investor and entrepreneur himself, just like Elon Musk. It's unfortunate that Romney included Tesla in his list of green losers - he did start this fight - but Tesla is the exception that proves the rule. The government has not come close to making money on its green energy bets, and will not even if Tesla is a massive success. It has lost $535M on Solyndra, $249M on A123, and $400M on Abound Solar. Even if Tesla is profitable enough to pay back its $465M loan, this is a very poor risk-adjusted return. Many of the other companies in the green energy portfolio are reportedly quite sick as well; we shall see what happens to the 26 that received loan guarantees in five years.
The dismal history of Russian and Chinese state-owned enterprises should be sufficient to give us pause, but the reasons that people oppose government funding of companies like this are several fold:
1. The government has the power to regulate or tax
competitors into submission. Regulators have strong
political incentives to begin colluding with
funded companies at the expense of the consumer to show
that the investment was a "success". This can be done
without fingerprints by doing things like holding
competitors to a higher bar when applying for permits.
2. Unlike the funds pledged by an LP, tax dollars are
extracted coercively (via threat of police action for
failing to pay the IRS). Arguably such acts should be
kept to a bare minimum.
3. The federal government is notoriously poor at being on
the cutting edge of technology; it is unlikely that they
will make better investment decisions than professional
investors.
One can go further, but it is striking to see how companies like Uber or AirBnB run into political roadblocks, while other companies receive $500M loans. The government is not just any old company or investor, the government has guns and can compel action in a way that others cannot. As such it is very powerful to have as a investor and very dangerous to have as a competitor. Too dangerous.