It's completely understandable that long-time LP members feel used, abused, and taken advantage of by former Republicans wanting to use the party for ballot access without an intention of sticking around, especially after experiences like Bill Weld and Bob Barr.
A late entrance to the race doesn't do much to inspire confidence on any of these fronts. But if Amash's strategy is to dive right into the general (probably prudent politics), the late entrance is logical. He hasn't gone to state conventions or participated in debates, which may seem like a snub to delegates.
On the other hand, that means he hasn't had to field questions from members or fellow candidates about abolishing driver's licenses and the US Dollar, disbanding the entire military, legalizing heroin, or amending age of consent laws... much less whether 5G GM vaccines are spraying alien chemtrails out of lizard Bill Gate's patented coronovirus centers for the purposes of population reduction and control.Right or wrong, it's immensely helpful to being considered a "serious" candidate for President by much of the media and voters, that one has held elected office. There has never been a Libertarian President, or a Libertarian US Congressman (until Amash). Even LP state congressman were elected as major party candidates and then switched... only to lose their re-elections running LP.
When it comes to how former recent Republicans that have run under the LP banner and how well their philosophies align with basic libertarian principles? When it comes to how well they are able to sell classical liberalism, the NAP, libertarian philosophy, or the concepts underpinning the writings of people like Hayek or Bastiat? Amash>Johnson>Barr, so at least there's constant improvement.