The biggest problem with impeachment, as I see it, is that even if it succeeds, it fails if Trump is reelected next year. Because the Constitution does not say a single-term president who is impeached cannot serve again if he's elected again. A rational Supreme Court would tell you that is implied; but thanks to 2016's protest-voters and stay-homers, the Supreme Court is no longer rational, it is now heavily partisan and is voting along party lines, and will unquestionably allow an impeached Republican president to return to office - do any of you seriously doubt it? And then Dem voters will be so demoralized at so dramatically snatching defeat from the jaws of victory that the GOP will win back Congress in '22. So, again - if impeachment succeeds, and Trump is reelected immediately afterwards, our government's vaunted system of checks and balances will now have no more credibility and we descend further still into a fascist autocracy.
If, on the other hand, impeachment proceedings are not finished before the election, and Trump is re-elected, the issue is moot because continuing with impeachment proceedings in that case can't be seen as anything other than a direct attempt by Congress to subvert the legal results of the election, it is an unjustifiable means even to the end of getting rid of Donald Trump.
And neither of those two problems can be waved away with "but Donald Trump won't win next year", or even "Donald Trump most likely won't win next year". Anybody who says those words with anything other than irony after what happened in 2016 belongs in a nuthouse, to be frank. That statement's just plain not true until it actually happens.