Are local GOMARs and Article 15's a career ender for Officers?

I would have to say it depends on the infraction, will it be applied to your OER, and how much scrutiny will be placed upon you by the Chain of Command. A local reprimand or Art. 15 will not follow you, what will follow you is your OER's, and people you serve with. An awful lot depends upon how the officer in question rebounds from this. If the officer takes the self pity route, or the outraged route, it could have negative repercussions, if instead they take the setback in stride, acknowledge mistakes were made, learns from it, and doesn't make the same mistake again, I see no reason why it would end a career. The Army usually attempts to rehabilitate SM's that make small mistakes.

If I remember locals don't because it doesn't follow you ( especially when you PCS) as its not part of your OMPF.

As far as an article 15. I think this is a good thread to read:
https://www.rallypoint.com/answers/in-today-s-military-can-you-survive-an-article-15-or-letter-of-reprimand-and-still-have-a-successful-career

Desire to know more intensifies.

If the GOMOR is placed in the MPRJ rather than the OMPF, they have little effect beyond the current assignment. Article 15 is a different story. The commander is required by regulation to place the results in either your Restricted File or Performance File. If it was placed in your Performance File, that information will be available to your next commander and to promotion boards and your career in the Military is most likely going to be short. Your current rater can, maybe should, note the Article 15 in your OER, even if it is in the Restricted File and again, it tends to be a career killer.
BTW, I am not totally convinced that promotion boards don't look at your restricted file. Probably paranoia on my part.

If a command is willing to go through with a Captain's Mast (Art 15), then they are not your friend. Competition of promotion is so tough that certain words entered in an OER will kill promotion. I replaced an Ensign (O-1) who went to Admiral's Mast (Art 15) for his failure to manage a ship's store. The PO1 (E-6) who worked the store stole thousands of dollars without the Ensign catching it. It made Ltjg, mainly due to timing, but did not get promoted to LT (O-3).

Not necessarily. If the individual messes up again and it becomes part of their permanent file or they are used to punish that person for messing up again absolutely. Also, not sure the legality but the offense may be mentioned in the OER which could be a career killer depending on how it is worded.