To be a little provocative, headspans are widely used in Germany and this has been the case for decades. Now, train reliability there is far from perfect but the problems that occur are those of network congestion, trespassers, train failures and seldom OHLE issues. So headspans can be made to work.
So one of the issues with headspans is that the catenary, the pantos and indeed the track have to be well-maintained to keep the contact within tolerances (including tension) and prevent dewirements etc. etc.. Generally, DB seems to manage this but perhaps less so in the UK? (Years ago, the consequence of not doing was brought home to me with a dramatic loss of pantograph incident on the viaduct near Hertford North: the panto bounced up and down on the contact wire and eventually ripped off the train (speed at c.40mph) with a dramatic bang and arc. The train train managed to cruise into the station.) So one of the reasons for going for the portal approach was, it seems, to mitigate against maintenance issues... but the answer from a total quality perspective should surely have bene to improve the maintenance first, ie prevent problems happening rather than mitigating them! Discuss....