when you construct a mining complex on a planet with a resource that has a tier other than 0, what you end up with is a regular levelable planet that happens to also produce ore, and be capable of shipping it if you so wish. however, if you build it on a T0 or a scalable resource, the ore resource takes priority, blocking the other resources when it comes to planet level and, in the scalable resource's case, higher level bonuses.

Here I have two screenshots of a planet with FTL crystals: https://steamuserimages-a.akamaihd.net/ugc/913545907614261061/785B4DD8FFC87790A1044E8C79E8C3DD8C0E4F00/

as you can see, this planet has a level of 3, a max pop cap of 21, a pressure cap of 48, an income of -385 (ftl building maintenance lol), a loyalty of 18, and the FTL crystal reads as giving 2 FTL per second. at the top you can see that I'm currently generating 9.7 FTL per second.

Here's the screenshot for after the mining complex is completed: https://steamuserimages-a.akamaihd.net/ugc/913545907614261244/35896A0E488F5147523D1C74AFA503692F02544D/

you can see that the level of the planet has dropped to 0 and imported resources are completely ignored (except for dealing with pressure). the pop cap has dropped to 5, the pressure cap to 16, I'm now losing 1.4M, loyalty has dropped to 6, and the FTL crystal reads as only giving .5. at the top right you can see that I'm now only generating 7.7 FTL. I actually dropped by a full 2, .5 more than I expected.

basic non-food resource planets cannot be leveled past 0, and T1+ planets do not face any priority issues, so these planets can be considered safe to build mines on. Planets with food, however (such as organic capitals), can usually be leveled, but here the ore takes priority, preventing the planet from being capable of leveling. This isn't really a problem except in the case of capitals, where it becomes a large problem. Scalable resources especially, which you want to be high leveled for the bonuses they give, cannot have mines on them, or else the priority blocks the scaling resources.