A basic soundcard test setup will cover guitar style amps. I lucked on an EMU0404, and it has very good unmodified frequency response, and allowed me to have a good look at a Williamson response at both low and frequency, but it wasn't the only tool I had a need to use. Even that soundcard needed a fair bit of checking/modifying probes and getting adaptors ready and confirming achievable bandwidth and appreciating how hum was sneeking in and how to alleviate it, and arranging a battery power supply and a USB isolator.
And although I find REW is pretty straightforward to use now, it took a fair effort to grow the experience, as I needed to use an ASIO driver for high bandwidth, but was also swapping to cheap $1 soundcard for other amp testing and that changes the driver, and there are so many tweakable parameters to gain confidence with. Luckily, REW is a living product, and is getting better and easier to use. But yes it doesn't have a normal scope display, which can be a hurdle for some.
As the tool has to connect to high signal levels, due care is needed to make sure the amp has effectively a common ground for input and output signals, and that large signal levels are properly attenuated (ie. 10:1 or 100:1 cheap ebay probes can be fine), and that DC levels aren't probed without having additional capacitor coupling protection. It can be easy to fry electronics from accidental probing - but no different really from doing the same with vintage hardware test equipment.