It's easy to disguise abuse as constructive criticism though. It seems more trustworthy because it's easier to believe, but really, that's part of the danger of it, too: you believe it...when actually, you could be perfectly fine. It's very easy for it to lead to a psychological "scarring" or a pattern of constantly picking apart your flaws and striving for a goal of perfection that you'll always perpetually fall short of. It can lead to feelings of inadequacy, or "not being good enough" regardless of how hard you try. It can lead to feelings of inferiority, particularly in relation to the person offering "constructive criticism" (which it may not actually be despite being disguised as such, and even if it genuinely is, it can still be toxic when it's excessive...and whether it's excessive probably varies according to the individual). Often times, those corrections are actually just subjective preferences, also, and it's simply that the person isn't allowing you to be yourself because they personally prefer that you're some other way they like more instead.
Furthermore, constructive criticism that you mention here is essentially a form of negative reinforcement. People can give positive reinforcement corrections as well though, which saying nice things would be one possible component of / tool in. This is how cats are trained, actually, and negative reinforcement doesn't work on them the way it does on dogs. Despite this, the positive reinforcement is how you can train them to stop doing negative behaviors. I honestly think a lot of people work this way as well...some responding better to positive reinforcement, that is. I also think each of them may be more appropriate, effective, or suitable, depending upon the situation. For instance, you wouldn't use negative reinforcement / constructive criticism when people are being uncomfortably tense and awkward guests at a dinner party.
The danger of constructive criticism is that 1) it's difficult to tell what exactly is "too much" or "constant," (as "constant" may be one thing to person A, and another to person B, another to C, and so forth; it's subjectively defined, typically by the person's internal affective response, and a lot of people won't stand firmly on those boundaries, especially if accused of being obstinate or defensive or something by doing this) and 2) the other danger of it is that someone's honesty is easily mistaken for the truth, and a person who honestly believes it is such and gives a lot of constructive criticism may revisit the accusation and even make more 'constructive criticism' comments about the fact that the person does not receive it as truth. If the person caves or ends up believing it, there is the risk of both leaning on this person's judgment too much (at the expense of independence) and/or conforming to someone's subjective standards, values, and so forth.
By the way, this is a tactic that can often be seen being employed by narcissists who see you as an extension of themselves and mold you to their expectations, standards, values, views, etc. Just because someone is being "honest" (saying something they believe is true) doesn't mean they're more trustworthy, and if it's constant then it's definitely--not just a red flag, but a red banner, honestly. Similarly, just because someone uses a lot of positive feedback instead, doesn't mean they're not encouraging changes in the same way as the individual(s) giving constructive criticism.
