As the former MD of a will-drafting company, I think that I'm fairly well-qualified to answer this one ;-)
Most 'will makers' (as you call them) can provide as good a service, if not better, than most solicitors. However, many professionals (both solicitors and will makers) don't ask sufficient questions to cover all eventualities.
For example, Fred might leave his estate to be shared equally between his adult children Alf, Bert and Charlie. That's a very simple statement to write in a will but it doesn't cover the possibility of one of Fred's children dying shortly before he does. Alf is unmarried (and has no children) so, if he dies first, Fred might want his estate to be shared between the two surviving children. However, Bert is married with no children. If he dies first, does Fred want his share to go to Bert's widow or to be shared between the two surviving children? Charlie is married with children. If he dies first, does Fred want his share to go to Charlie's wife, Charlie's children or to be shared between Fred's two surviving children?
A properly written will should always cover the possibility of a beneficiary pre-deceasing the testator. (Ideally, it should also cover the possibility of a beneficiary only surviving the testator for a few months after the death of the testator).
You don't need either a solicitor or a will maker to write a will. You can do it easily, and legally, yourself. However, it's probably worth using the services of a professional to 'tie up the loose ends'. My recommendation is that you should ask at your local library for a copy of Wills and Probate, published by 'Which? Books'. Use the information you'll find there to draft your own will. Make sure that it covers all eventualities, then take it to a solicitor to check it.
Chris