This is true. There isn't actually any comparison between the two things. Because with expert witnesses, their professional career and how they're regarded in the field is the whole point. This is why you want, say eminent doctors and professors rather than someone who's just started out in the field, or a random member of the public you found in Tesco. It does actually matter a lot if they advertise themselves as working in a particular prestige role, and then they're not.
Barristers and solicitors aren't there for the same purpose. They need to meet the minimum standards required for those professional bodies, and as a client you'd also want to know if there were any ongoing proceedings with the regulator like with a doctor, but otherwise that's not what they're for. Even with those lawyers who are so eminent that they're arguably equivalent in reputation to the expert witnesses, the jury and court isn't supposed to take that into account.
I'm not saying the complaint would necessarily have turned out to be relevant either, but an expert witness should be candid and proactively disclose anything ongoing. He didn't. That's a problem. I have to wonder why Letby's defence didn't do anything with it.
I say this as someone who's clear that for all I know (and tbf I don't know anything about this case) he might be an unpunished rapist. They're everywhere and most of them never get near a trial. The legal professions certainly aren't immune. I have heard an accusation about one solicitor in his pre-practice days that I personally believe, for example. It would astound me if there weren't rapists at the Bar. There's about 17,000 practicing in England and Wales, men are the majority and even the lower end estimates of the prevalence in the population is going to give us an uncomfortably high number.