Traveline

What date are you looking for as I can't replicate this looking for a date in mid March. I've merely asked it for Cardiff to Brecon without specifying where in Cardiff or Brecon.
It's a 1hr 47 minute journey, so yes say 2 hours for ease.
Stagecoach (the operator) are not advising of any changes to the service on their website,
Traws Cymru is an umbrella organisation to co-ordinate all the long distance domestic bus services across Wales- integrating the various operators, timetables and fares into one network, and all buses should be in their colour scheme, so the operator isn't of importance to the general public. At Brecon a number of various services connect at the same time each hour.
Traveline Cymru is the welsh version of a series of government funded operations across the UK which provides integrated multi modal information across the area or across Wales and Scotland- train, local bus, long distance bus (like Traws Cymru), ferry, the domestic air service to Anglesey (before it was withdrawn) etc, and will add walking or taxi where no other option exists.
The two should always be telling the same story, so very odd that they apparently aren't.

TfW is the rail operator for all Welsh domestic services and almost all external services to England. They confuse the matter by talking about buses, which they do not currently run, other than as rail replacements when there is engineering work.

I would have thought the way to go would be train to Abergavanny then bus from there or train to Merthyr Tyfil then bus

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I'm not sure why the OP would want to do train to MT first if he can get a direct bus (the T4 calls at the smart new Merthyr Tydfil bus station anyway). That doubles the cost and requires a walk from MT rail to bus stations, and does not solve the supposed problem area between MT and Brecon.
Likewise at Abergavenny you have to walk between railway and bus stations, and it is only every 2 hours on the X43, so miss a bus due to a late train and you have a long wait, and again have doubled costs. Even with perfect connections it's no faster.
If I had to do it by train first I would go by train to Hereford, where the T14 stops at the station entrance, but it's a very long and expensive way round. I have actually done that route (T4/T14) from Cardiff to Hereford via Brecon with a 2 minute change at Brecon. It's a very scenic route, but a long old way.
It would probably be shorter to catch a Swansea train to Neath (where the bus stops at the station entrance) then the T6 to Brecon, but you would only do that if the T4 does have a big problem.
Like the X43 and the T14, the T6 is only every 2 hours, so is not an ideal answer.
The T4 if it's running, is by far the best and most efficient route, especially if the OP has luggage.

Thank you for the explanation of the different websites/companies. I just want to understand the options so I can choose the best routes. I am giving up on the brecon beacons for this trip. I have so much planned to see and do so Im fine with it. Thank you for responding!

Probably no use to Tvlnow if he/she is no longer including the Beacons, but just for future reference the best connection I have found for Brecon itself is train to Neath, and then the T6 to Brecon. Most, but not all, the T6 journeys go all the way to Brecon, so check the timetable. Avoid routing via Abergavenny as the train station is quite a long walk from the bus stop.

and then there is the question of the name of Brecon Beacons.

Apparently the CEO of the area has decided that the name is not politically correct because it indicates burning wood and that puts carbon in the air and we don't want that. So the name, in use for so long, is on the way out to be replaced by a name in Welsh more environmentally friendly.

I just report the news, I make no comment.