Sure, if you have a marketing budget you can do all kinds of stuff. Here, OP is marketing against the app which is already at the disadvantage of not having a marketing budget and being free (which people incorrectly associate with bad).

So.. You know, do the devs a favour and don't market against them? It doesn't harm you to just stay out of it if you don't have anything nice to say. Go dunk on the big corps. They can financially handle it.

You seem to think that 'marketing' equals 'deception to trick people into buying a product'. This is a very narrow view.

Marketing is a much broader field [1]. Of course a part of it is about how to effectively convey an idea to someone. However, deceiving people will only work for short-term businesses, as it is much more work to build your image after a deception or schandal. That is why brands often cease to exist after a schandal, as starting a new brand is much easier.

I developed an open source app myself and I'd rather inform future users about its limitations before they use it. This way, they have a positive feeling towards the app, even though it could not deliver value in their case. This has nothing to do with budget (for my open source project, budget is ~0 anyways). You need budget for advertising, reaching out to potential customers, engage with your customers, etc. The budget needed for clearly communicating the value of your product is negligible.

You seem to think that downsides or limitations are inherently negative. Knowing a downside in advance is actually a huge advantage.

[1]: "Marketing is the process of exploring, creating, and delivering value to meet the needs of a target market in terms of goods and services" - https://en.m.wikipedia.org/wiki/Marketing

Guns arent inherently bad, either.