Oh, well I missed when Trump was impeached for those things.
My mistake.
There are a few questions that may be asked when deciding whether to start pursuing impeachment.
(1) Is the act (objectively) serious enough to warrant impeachment? (The notion that there's any objective answer to this may strike some as ridiculous and others as plausible.)
(2) Can we convince (most of) the voting public that this is serious enough to warrant impeachment?
(3) Can we convince at least some Republican Congressmen of the same?
For (2), the act has to be one that most folk can understand and agree is bad. The Ukraine thing is just such an issue, not because poor widdle Biden was victimized, but because Trump is using foreign policy to gain political advantage in an illegitimate way. That's pretty easy to see is wrong, if you can convince folks that this is what happened. The fact that we have a knowledgeable whistleblower bringing the allegation, evidently a career civil servant (if that's an appropriate term for a CIA analyst) helps make this case.
Now, personally, I care more about (1) than (2) or (3), even though some here may scoff at the presuppositions for (1). Those who care about (3) (David Brooks and George Will are in that camp, as are Trump defenders and some Democrats) primarily would say that this isn't the time and there probably is no time for impeachment, since there is no hint the Republicans will cave on any issue ever. This is probably too cynical and perhaps once the evidence comes out, some Republicans will rediscover a spine.
So I wouldn't say that attacking Biden is a step too far. It's not really about Biden, far as I'm concerned. If it turns out that Biden really was illegitimately interfering in Ukraine in order to protect his son (which seems terribly unlikely given what we know), I'd like that to become known and for his political career to be over. But I'd also say that Trump's actions deserve impeachment because he has not acted in the interest of the U.S. or justice but in his narrow political interest.