To or from Heathrow by Megabus?

What sort of thing do you need to know? It's a fairly standard coach service that takes from 2h35m to 4h15m depending on the time of travel. Personally I would take the train every time rather than be couped up in a coach, especially if the option was a slow coach. The one plus it has is the price as Megabus do this for under £20.

There is nothing wrong with Megabus, but
1. Whenever they can Megabus (and Flixbus) depart from on street bus stops, often with minimal if any signage- rather than from bus stations (with their greater facilities like National Express do)
2. Megabus and especially Flixbus make fewer rest breaks than National Express.
3. At Heathrow Megabus stop only at Terminal 2/3 (Central Bus Station). From South Wales National Express call additionally at T5, from many other destinations also at T4.
4. National Express are very similar for fares to Megabus from Cardiff, on many routes cheaper and in terms of age and comfort of the fleet NatEx are more dependable. Also from Cardiff NatEx are far more frequent.
5. If you are over 60, NatEx will give you 1/3rd off their fares, on purchase of a £10 coachcard, and on Tuesdays to Thursdays have a maximum £15 fare for coachcard holders.

For ease of use I would prefer coach than train for South Wales to Heathrow, for a broadly similar journey time and to avoid changing trains, potentially twice depending on route taken.

Thankyou @isn31c and @jb**
I was originally planning to travel by train, but I've had a mobility set-back and friends have offered to meet me, so have switched to coach as involving less walking. The NatExpress website won't function properly on my legacy computer so I switched to Megabus. The street drop-off should be OK in Cardiff, but I'll return from Newport where I can wait at the familiar train station.

Fyi...you can take a National Express coach direct from Newport to Heathrow. No changes.

Some are even nonstop.

The other thing to consider is traffic, but since you will not be going into London proper, it really should not be a big issue.

I took a National Express coach from Portsmouth to London Victoria, to avoid any issues on a train strike day. It was pleasant, until we hit the metro area. delayed a good 30-45 minutes in slow to stopped traffic. The train, if available, would have been much better.

However, our Heathrow to Bristol coach was direct, prompt, pleasant. If you were to take National Express, that would likely be the same route for you (the bus continued on to Cardiff).

Just a caveat on travelling out of London towards Heathrow and beyond to Cardiff/Bristol..I’m currently living in the Southwest but commuting to and from London by coach. Peak time, esp Friday afternoons, it can take over two hours to get from Victoria as far as Heathrow. You need to remember to factor that in as the coach drivers won’t let you hop off and get the tube.

Thanks for your concern @monkfishface100, but I'm travelling westward from Heathrow, and not going near London. And at off-peak hours on a Tuesday.