I don't really find the transit experience at TPE that different from NRT or HND. Never used ICN so I can't say. HKG is slightly better but mainly because it is a CX megahub so you are just staying within the CX ecosystem. I may have a different take if I ever try to do CX to non-CX transit.

Immigration at TPE is pretty efficient and the experience is good. Never really had any problem when entering Taiwan. The experience in general is better than entering Japan at HND which has horrible lines and very bad way-finding signs.

TPE CI or BR lounges are decent or fair... have not been to JX lounge. I'm used to the pathetic lounges at LAX so I think TPE lounges are pretty nice But clearly, it is not SIN. But compare to HND JL or NH lounges for example, I think the crowd size and amenity are about the same. CX lounges have better food choices for sure, especially at HKG.

Exterior of TPE is... unimpressive to put it mildly. Typical of a lot of infrastructure originally built in the white terror KMT authoritarian era, they were strictly functional and not allowed to have any design aesthetic or flourishes that could be deemed politically sensitive or provocative.