Is this for speaker amps or headphone amps?
OTL speaker amps tend to be a bit of a sticky topic on a lot of tube DIY forums. There are not a whole ton of tubes that are all that happy passing the kinds of currents you need to drive a loudspeaker, so you end up needing a ton of valves to get the job done. It's also tricky to maintain control of the current each valve draws without compromising the performance of the amp, and most often you'll see a generous lump of global feedback wrapped around these circuits to lower distortion and provide adequate damping. On the positive side of OTL amps, the parts cost is quite low and there aren't any complicated audio transformers to design and manufacture.
On the other side, SET amps typically use far more linear valves compared to those in OTL amps, far simpler circuits, usually a simpler power supply, but they require output transformers that are suitably made for audio performance, which isn't easy. With the output transformer, however, the same amp can be used on a wide variety of different speaker impedances by adjusting how the speaker connects to the secondary of the output transformer. To do this with an OTL amp isn't practical in any manner that I can think of.
If we are talking about headphone amps, typically OTL amps are a bang for the buck product that will synergize really well with higher impedance headphones that work well with an amplifier that isn't voltage limited. A good transformer coupled amplifier will typically outperform any OTL amplifier, but again this comes at the cost of highly specialized custom transformers that just aren't that easy to obtain.
