Learn Hardcore Russian: Не стоит увеличивать бюджет проекта без причины. - It’s not worth increasing the project budget for no reason.

сто́ит: stress on о
увеличи́вать: stress on и
бюдже́т: stress on е
прое́кта: stress on е́
причи́ны: stress on и́

This is an impersonal construction. (Не) стоит + infinitive expresses general advice/assessment: it’s (not) worth / it’s (not) a good idea to…
Because it’s impersonal, Russian doesn’t need an explicit subject like you/we/one. The “doer” is understood from context, or it’s meant to be general.

What exactly does не стоит + infinitive mean, and how strong is it?

Не стоит + infinitive usually means it’s not worth doing / you shouldn’t (really) do it / it’s not advisable.
It’s often softer than a direct command like не увеличивайте and a bit more conversational than не следует.

Why is the verb увеличивать (imperfective) used instead of увеличить (perfective)?

With не стоит, Russian commonly uses the imperfective infinitive to talk about an action in general (as a practice or idea).
Не стоит увеличивать бюджет… = don’t make a habit of increasing it / it’s not a good idea to increase it (in general).
You can sometimes see не стоит увеличить…, but it tends to sound more like a one-time, specific action in a particular situation; many speakers still prefer imperfective after не стоит.