On one hand, I think this kind of service can help people expand their vocabularies.
On the other hand, there's a downside risk of replacing unimaginative writing with confused writing. "very + fragile -> feeble" is absolutely not a reliable translation[1]. The suggestions furnished by this service seem to generally 'work' in the sense that their possible meanings include "very + (category)," but without context you explode the set of messages you might communicate. Something that is fragile breaks easily. Something that is very fragile more so. Something that is feeble is lacking strength in general, it may give out at any time, it connotes a sense of being underweight, etc. All concepts that "very fragile" may be hoping to avoid invoking (however clumsily).
Switching words switches meanings.
[1] One might argue that 'fragile' does not need assistance at all in this case.
Regarding your [1], I think people reach for "very" for two different reasons: because they habitually "overseason" their sentences, adding intensifiers when the underlying language is already intense, or because they "underseason" them with limp adjectives that they're trying to pump up. The former people just need to build their confidence to drop "very". The latter need something like this site to help learn better adjectives.
I think my point is that these adjectives are not "better" - they are different. They mean different things, which you might also mean, but aren't good "drop in" replacements.
On the other hand, there's a downside risk of replacing unimaginative writing with confused writing. "very + fragile -> feeble" is absolutely not a reliable translation[1]. The suggestions furnished by this service seem to generally 'work' in the sense that their possible meanings include "very + (category)," but without context you explode the set of messages you might communicate. Something that is fragile breaks easily. Something that is very fragile more so. Something that is feeble is lacking strength in general, it may give out at any time, it connotes a sense of being underweight, etc. All concepts that "very fragile" may be hoping to avoid invoking (however clumsily).
Switching words switches meanings.
[1] One might argue that 'fragile' does not need assistance at all in this case.