Last hing you want is a stone kicked up and getting into the cambelt. Low probability but high cost if it happens.
The TA has a fully enclosed chain driven camshaft, so there's no real risk of a passing stone causing any significant damage.
There
is a small risk on the 1.2 models, which have a cambelt, but unless things have changed recently, 1.2 models have no underbonnet tray - it's only fitted to the TA.
I'd expect models sold as having off road capability would have a more robust form of underbonnet protection that the usual flimsy plastic tray.
Sadly many vehicles originally fitted with underbonnet trays seem to 'lose' them during routine servicing. Worth checking before you drive away after it's been in the workshop.
When I finally sold my R5 after just shy of 220,000 miles, I was proud that it still had its OEM underbonnet tray and original bolts, all in good order. Most disappeared after the first service.
.... but this doesn't clear the clag gathered at the bottom of the sump around the (hopefully) magnetic plug.
The tapered plug used in the 1.2 isn't magnetic.
With modern detergent oils, there shouldn't be any clag around the sump plug if the oil is changed at a sensible interval; I've certainly never noticed any with mine.
Suction may actually get more of the old oil out than conventional draining. My real concern would be the cleanliness of whatever they stick in the dipstick hole; this shouldn't be an issue if you're doing it yourself.
The same goes for the coolant, especially on those models with an expansion tank that's integral with the radiator. Removing the bottom hose won't get you any more fluid out (guess how I know this?), and offers limitless possibilities to break something fragile and get into difficulties.
