Fritz has pretty well summed up the "short" answer.
I and my buddy have HK products so I am not disparaging something from the "outside". The long answer is that the HK community is very polarized, and as far as many of them feel, HK clones don't deserve to be called "HK".
The clones - bobcat, special weapons, etc. are mostly from parts from one source, "Todd" of special weapons. You can search around and find information about him, and make your decisions from there. I have dealt with them several times and have not had any problems from a business perspective.
I have owned the G36 conversion parts and built mine from an SL8. At the time, I moved and needed money for the house deposit, so I sold it. The conversion parts were from special weps, and honestly I had no problem with the parts or the service. Many people have reported issues, however my experience was different. I would personally not convert another SL8, even if they were available and affordable, without a major redesign of the conversion parts. The conversion essentially worked "with" the factory "semi-only" receiver. The true G36 parts could not be installed. The conversion needs a serious amount of work to be done "correctly" and there is some question as to whether this is even doable with the plastic receiver. With the US-built conversion, the length of pull is about 3" too long. It is therefore going to feel strange unless you have a large frame. Swapping the short parts onto it will prevent you from shooting it with the stock folded, which to some is important. I didn't care for the balance of the G36 either... it felt out of balance.
There are some UMP conversions that use a shim to use original HK parts and conversions for greasegun mags. Now THOSE look interesting. However there are other .45 ACP rifles that I would consider before I invested all the work into an UMP conversion... the Kriss, a good Tommy Gun, etc. There are several other .45 rifles that IMNSHO are superior to the UMP.
The Vector weapons are unanimously considered to be "the" builders of superior HK clone products. There is another one, PTR I think? Those are also top-shelf. I would not hesitate to own anything from either of these two companies. Keep in mind they will still be considered "HK clones" from some collectors' viewpoint.
A few thoughts -
As Fritz said, resale value will be lower on non-HK rifles for some HK collectors. By that, I mean *ANY* government contract rifle from Pakistan, Portugal, or any other place that builds HK rifles under contract. Some in the HK collecting community look *ONLY* for parts that say "Made in Germany" on the side. Case in point, my Portugese-built G3, using HK equipment, has a slightly lower resale value than an HK-91, which says built in Germany. Food for thought.
Then again, I'm not a fan of their pistols. My .40 USP pistol had a terrible trigger out of the box, and needs a trigger job. It is heavy and rather large for a "compact". It drives tacks, to be sure, but it just feels "cheap" in how they made it.
Something to consider is that parts are extremely expensive for HK stuff. Other than G3 magazines

There is no reason, except they say "HK" on them! Case in point, my HK G3 heavy bipod? > $400. (!!!) Look at the handguard-mounted bipods and you can find the German ones for around $150, and the "contract" ones for around $50. You can usually find knock-off stuff like Asian mags, etc. for cheaper. Other than the factory scope, which I'm not interested in collecting, my collection is "complete" and I invested around $2500 total. That's counting the mags, which I bought *BEFORE* all of the cheap $2.00 mags were available. I think I spent $11 per mag... around $200 total. Sucks when I think I could have saved $180 by waiting a few years.
Is it worth it? You'll have to make that decision. If you are trying to create a "museum quality" collection, you may choose to buy only "pure" German rifles. However the discussion doesn't really apply to other rifles, because no one is going to throw away their AMD-65 because it is not a "pure" Russian AK. See my point? I like my G3, my buddy likes his, and frankly I don't care that mine wasn't built by someone speaking German vs. Portugese. Mine is an actual battle rifle and that's good enough for me.
ETA, I had a similar "we are the only people with good guns" experience with my Colt. The gun my bro built is based on a Model 1 Sales upper, and he has had none of the problems some people say they've had with the Model1 parts. And he has rode it hard. Especially after I gave him my C-MAG drum

His rifle was every bit as good as my 6920. Those seem to be going back up in price, so perhaps it would be a better thing to hold onto the Colt for a bit longer.