My understanding is that a strong majority of scholars (including conservative scholars) take the position that the long ending of Mark was not in the original and was not written by the same author as the rest of the text, but nonetheless was added very early on (probably in the early 2nd century). However, the evidence is not as overwhelming as for the Comma Johanneum or the Pericope Adulterae, in part because the long ending of Mark is significantly older than the those two.

The main arguments are as follows. Certainly people dispute some of these arguments, but on the whole each of them is a strong argument, and taking several together gives an even stronger argument.

The long ending does not appear in several of our earliest and best manuscripts, most notably Sinaiticus and Vaticanus (although it does appear in Alexandrinus).

Many early manuscripts which do contain the long ending nonetheless contain indications marking it as disputed.

The existence of manuscripts containing a different ending entirely (the "short ending") also suggests that the original contained no ending.

The author of Mark has a distinctive Greek style, and the long ending does not match this style.

The author of the long ending appears to be familiar with possibly Matthew, probably Luke, and possibly Acts, while the author of Mark was not.

The authors of Matthew and Luke do not appear to have had the long ending in their copies of Mark.

The textual evidence, which covers the first three arguments, as found in Nestle-Aland is summarized ably at the end of the Wikipedia article under "Summary of manuscript evidence." At any rate all of these are largely undisputed facts

Is the ending of the Gospel of Mark (16:9-20) original?