An excellent hot-side film coefficient can still sit inside a mediocre heat exchanger — because it's never the best individual h that decides overall performance. It's the worst one.
An individual film coefficient (h) describes convective heat transfer through just one resistance in the path — hot fluid to tube wall, or tube wall to cold fluid — measured or estimated separately for each side of an exchanger. The overall heat transfer coefficient (U) combines every resistance in the full path in series — hot-side film, wall conduction, any fouling layers, cold-side film — into one single number via a resistances-in-series reciprocal sum. U, not any individual h, is what actually goes into the exchanger sizing equation Q = U·A·ΔTlm. That single fact is why a heat exchanger with a superb hot-side film coefficient can still underperform badly: if the cold side or a fouling layer is the largest resistance in the chain, it dominates the sum regardless of how good the hot side is.