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There is very little major difference in the final power devices for ATSC and NTSC. However, there are details of the plumbing and sizing of the devices.
In NTSC, the max power is during sync, and some transmitters used pulsing of the outputs to reach peak sync level. This won't work for ATSC, which must be scaled to make the peaks at random times as required by the digital waveform. These peak values are mostly within +6 dB from the average power. Some peaks may go higher, but it is not crucial if they are compressed slightly, as long as the signal passes the EVM limits explained below.
Whereas analog transmitter performance was measured in frequency response and non-linear effects on the chroma carrier (differential phase and amplitude), digital transmitters are characterized by the "error vector magnitude," the deviation from ideal, measured as the ratio of signal-to-(noise+distortion) in the output, as determined by a standard test decoder.
Digital transmitters also have much stricter out-of-band radiation limits than analog transmitters, to allow the closer packing of stations within the available spectrum - so they have more complex baseband, IF, and high-level RF filtering. In fact, the FCC "mask" is so tight that most test instruments cannot measure the over-all result, which must be measured in two steps: 1) the raw transmitter output, and 2) the filter attenuation.
I believe tube finals are still in vogue for high-power UHF.
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