Lance,
Net exchange efficiency in a heat exchanger comes from a variety of factors, including but not limited to, surface area (cross sectional area of transfer), conduction efficiency (heat transfer coefficient) of the exchange separator, minimizing pressure drop, minimizing heat loss, and flow type characteristics (cross flow, parallel flow, counter flow).
Never mind these particulars for a moment, as any specific type (e.g. Spiral coil, straight shell and tube, plate type) can all be configured for adequate efficiency for our purpose. However, in our application straight shell and tube can have advantages and disadvantages.
A coil type is often considered cost effective, for simplicity sake, as applied here. In order to increase heat efficiency though, one might have to introduce characteristics which do not led itself as readily applicable. By using narrow tubing to increase efficiency through adding surface area of transfer, certain applications which might present fouling issues might make for both clogging and/or reduction in usage efficiency as fouling occurs. If we use chemicals which are introduced before heating, then these chemicals can present a fouling problem. Hard water can also precipitate minerals which can deposit on the coil material. A little fouling reduces heat transfer, a lot of fouling can actually lead to a clogged tube. The coil-type exchanger on your Performer is of a relatively large diameter and uses multiple internal coils with headers that reduce the effects of this clogging at the expense of surface area and heat exchange efficiency if the coil wall thickness is needed for high pressure application. However, those heat exchanger designs that use a single small coil for efficiency of perhaps ~3/8" ID run a real risk of a "catastrophic" clog if not carefully tended for clear operation (descaling). Once clogged, a single coil is practically impossible to clear, and the HX is "toast".
Multiple coils attached to a header, or better yet, straight tubes offer the user the ability to run with several clogged tubes, and only lose efficiency, not the whole exchanger. Moreover, if those tubes are straight (such as in Harper's linked video), you can actually clean fouling by passing through a physical "rooting". Moreover, if the tubes are welded to the header bulkhead, can be welded shut if one or more tubes develops a leak. If a coil type exchanger develops a leak, you have to hope that the exchanger is serviceable enough for dis-assembly to repair a leak.
I'm sure there is much more debate to the question than I have outlined, but I am limiting this to HX units seen in application for TM's, by and large. In general, I have found the straight shell and tube exchangers to be more serviceable than simple coil type most in evidence. Your coil type, as used in the performer is a notable exception. It uses large diameter, multiple coils, with internal headers, and is disassemble-able.
(Marty Sutley approved type summary) Below
Most of the higher end HX units out there that use coil type are made by manufacturers who have an interest in keeping any possible disadvantages of the coil type to a minimum. It's the lower end machines that simplify the coil design to "squeeze out" minimum performance at he expense of serviceability that represent the potential issues described. The straight tube and shell are just much less apt to exhibit these, and afford compact design with minimal pressure loss.
