[mulveling]

Advantages of using a SUT:

• Lots of noise-free gain, usually in the range of 20dB (10x) to 32dB (40x), to replace an active MC gain stage (i.e. the MC stepup, or headamp stage) that necessarily contributes to the noise floor.
• Allows tube purists to use an all-tube chain for low-MC cartridges, without paying the toll of high noise floor from having all gain stages be active. I.e. if you find a SUT you love with your low-MC then you need only look for active MM stages to complement it; note that there are very few phono stages that utilize all-tube gain stages and support low MC's, and they're (without exception) notoriously difficult to source ultra low-noise tubes for. And the more gain you can get from your SUT, the more it will effectively suppress noise floor from your MM stage, because your line-stage's volume control can be turned down to reject that noise.
• Just as some people love the sound of vinyl or tape, some love the sound of transformers with MC cartridges: "a coil driving a coil"
• There are absolutely superb sounding modern SUT choices for prices that are very reasonable in context of today's hifi analog gear; i.e. $300 - $1600. CineMag and Lundahl have many great choices in this range; Jensen is also quite popular and very affordably priced (though I haven't heard a Jensen MC SUT yet, their line-level transformers are excellent).
• A SUT is a passive device, so there's nothing to break, burn out, or replace.
• Each SUT model has a distinctly different sound, and synergizes differently with various cartridges -- so welcome to another level of audiophile rolling nervosa! You may be shocked by how different various models sound. You will obsess over amorphous vs. mu meta cores, silver vs. copper wire, which ratio is optimal for your cartridge, how to load (or whether to) the secondary (and how exotic you go on those resistors, e.g. nude Vishays), etc.

Disadvantages:

• SUT's have their own distortion artifacts. Typically it gets worse as you increase the step-up ratio (i.e. as you increase gain).
• The interconnect from SUT to phono stage (MM input) becomes MASSIVELY important. Keeping it short and very low in capacitance is crucial. Low-capacitance phono cables are are good choice here. Massive audiophile-grade large-gauge interconnects can be BAD. You might be shocked by how much difference this link can make.
• Besides the extra interconnect with an outboard SUT, you also introduce 2 extra connection points (1 contact, 1 soldered) for the very delicate, very low-level analog signal to traverse. It's always ideal if you can have the SUT built into your phono stage -- which is exactly the approach of many tube-based phono stage makers.
• You may have to do some research and fiddling with grounding schemes to suppress all the hum/buzz gremlins.
• Unless you are a DIY'er or having your SUT interface custom built, you may not have great loading options available -- in that case, the load the cartridge sees is purely a function of the stepup ratio: MM stage input impedance (almost always 47K) divided by the SQUARE of the ratio (e.g. 40x => 47K / 40 / 40 = 29 ohms). This can lead you down a path of having multiple SUTs if you run multiple carts.
• SUT's can be awkward little lightweight boxes to situate; heavy cables often tip it, and you must consider vibration control because it's relatively closely connected (via cables) to your tonearm.
• Each SUT model has a distinctly different sound, and synergizes differently with various cartridges -- so welcome to another level of audiophile rolling nervosa! You may be shocked by how different various models sound. You will obsess over amorphous vs. mu meta cores, silver vs. copper wire, which ratio is optimal for your cartridge, how to load (or whether to) the secondary (and how exotic you go on those resistors, e.g. nude Vishays), etc.