What the extra width gets you
An ultrawide keeps the height of a normal monitor and adds width. A 27-inch 16:9 monitor is 23.5 inches (59.8 cm) wide and 13.2 inches (33.6 cm) tall. A 34-inch 21:9 panel is 31.3 inches (79.4 cm) wide and 13.4 inches (34.0 cm) tall. That's 33% more width at the same height.
The 49-inch super ultrawide goes further. It's 47.2 inches (119.8 cm) wide. Two 27-inch monitors side by side add up to 47.1 inches (119.5 cm), so it's the same width without the bezel gap in the middle. Check that your desk and monitor arm can take that width before you order one.
Sharpness, pixels and cables
A 34-inch 3440 × 1440 panel has 110 PPI and a 49-inch 5120 × 1440 panel has 109 PPI, against 109 PPI for a 27-inch 1440p screen. The pixels are the same size as on the 27-inch screen, so text looks just as sharp and there's simply more room across.
Every pixel has to be drawn. A 3440 × 1440 screen has 4,953,600 pixels and a 5120 × 1440 screen has 7,372,800. That's 34% and 100% more pixels than a 2560 × 1440 monitor, so your graphics card has more to draw.
The cable carries the extra width too. 3440 × 1440 at 165 Hz with 10-bit color needs 27.2 Gbps, and DisplayPort 1.4 carries 25.92 Gbps. That mode needs DSC on a DisplayPort 1.4 link. The Display Bandwidth Calculator checks your own mode.
How games treat the extra width
The picture only gets wider if the game supports the shape. Most games use Hor+ scaling: they keep the vertical angle fixed and show more on the sides. Some use Vert- scaling and cut the vertical angle to make up for the width. A game that stretches the image to fit distorts it.
With Hor+, a game with a 60 degree vertical field of view shows 91 degrees across on 16:9, 108 on 21:9 and 128 on 32:9. Each step out gives you more of the scene to the sides. Look up a game's ultrawide support before you buy for it.