Incentives everywhere to overstate Trump's power
President Trump's political style is classic populist strongman. As such, he is going to explain politics as a set of illegitimate institutions setup by corrupt elites, which require nothing more than a strong leader with the will to rein-in the institutions, throw out the elites, and replace it all with obvious, virtuous policies. Such a leader isn't common but, luckily, he alone can do it. He can never deviate from the maximal power sales pitch; to do so would be to admit the impossible, and shatter the entire persona. He's always right, and he always wins.
Partisan allies of the president have strong incentives to promote this view of power and the presidents ability to be the unbeatable transformative champion, even if they don't actually believe it. Some of this is wider than Trump; co-partisans are never interested in embarrassing the administration, and presidents are usually given some latitude as party leaders to set agendas and shape party messaging and style. So you get a lot of sycophantic coo-cooing from House Republicans about how all-powerful Trump is and how great the administration is in everything they do.
Trump in particular has also shown a willingness to go to war with partisans who dissent, and has managed to run more than a few people out of town. So even if you can't stomach going out and parroting Trump's story of politics, the smart play is probably to just keep your mouth shut. That's no particularly heroic, but it's also a reasonably smart political choice.
Partisan opponents of the president have arguably more incentive to portray Trump as an all-powerful wrecking ball. If you want to sell Trump as bad for the country, it helps if he's seen at least in some sense as capable and effective in his evil! And so Democratic leaders paint a picture of Trump as steering US policy off a cliff every time he touches something or other. Whether or not that's true, it sure sets up the underlying concept that Trump can at least destroy stuff pretty easily.
Partisan media on both sides have obvious incentives to magnify all of this, since viewership is highly partisan and receptive to the message. Trump-as-savior sells on Fox. Trump-as-devil sells on MSNOW. The unifying feature, however, is a requirement that Trump is powerful.
Consequently, it's very hard to get an unbiased view of how effective the president is at securing durable policy change. Everyone is incentivized to overstate it in the same direction! Even if Trump loses more than a typical president, the message you are going to get from all angles is something like the exact opposite.