Does it actually matter?
Yes, it often matters with certain kinds of adverbs, but the cases you have presented here allow for some flexibility.
Adverbs of manner (
worryingly, better) usually occur after the verb, so probably the most usual positions for your examples would be:
has increased worryingly over the last years
to understand better what is being said
Nevertheless, in spite of the split infinitive,
to better understand seems a rather usual way to say it as well.
On the other hand,
has worryingly increased sounds awkward
stylistically. (Too many syllables between the auxiliary verb and
the lexical verb, perhaps?) But there is nothing wrong with the
grammar.
CJ