Disclaimer: I am not a lawyer. This should not be regarded as legal advice. If you require legal advice, I recommend you seek the counsel of a bar certified attorney, or equivalent licensed professional for whatever jurisdiction you call home.
TLDR: The short version of the below is that I believe at least fifty percent of Free Radical's answer is mistaken or imprecise, and the COIL fits more closely with the PD/MIT/BSD group than any of the other licenses in that list, apart from the fact the COIL comes with a patent clause protecting recipients from patent litigation.
As noted by both MSalters and Wolftune, it appears that Free Radical has made some errors of interpretation. The COIL does indeed contain the language "subject to these conditions", and does indeed provide for sublicensing. Free Radical's explanation seems self-contradictory on the point of conditional licensing, in fact, where it says abiding by the license terms is not a precondition for the grant of license, just after saying that the license applies "provided that the user retains applicable copyright or other legal privilege notices, these conditions, and the following license terms and disclaimer." In my estimation, the "covenant" versus "condition" complaint raised by Free Radical should concern you exactly as much as it does for the MIT/X11 License or the Simplified BSD License (which is to say: probably not at all).
The concern Free Radical shows for license "enforcement" is largely inapplicable, because the license is (apart from requirement for copyright and license notice requirements) composed entirely of permissions, which requires no enforcement, as Wolftune pointed out. I further agree with Wolftune's assessment of how to classify "strong" versus "weak" patent terms in the context of granting permissions/license to recipients, and with MSalters' assessment in the context of the kinds of guarantees for the recipients of the licensed work. While I have no particular objection to a patent termination clause strictly as an anti-aggression provision, I also do not think that the lack of such a thing particularly devalues the patent clause itself.
Regarding GPL compatibility, it is worth mentioning that the Apache License 2.0 (AL2) is incompatible with every version of the GPL except GPLv3, and Apache License 1.0 is incompatible with all versions of GPL. In addition, AL2 and AL1 are mutually incompatible. Meanwhile, as Wolftune points out, COIL should not in fact be incompatible with any version of the GPL for a number of reasons, including the fact that it does not contain copyleft provisions (that is, copyright license terms affecting any parts of the work other than the parts explicitly covered by the COIL itself). Free Radical's primary claim for license incompatibility seems to center around patent conditions, but GPL incompatibility on that, I think, would have to involve the GPL's requirement that other licenses combined with it cannot be more restrictive than the GPL, and the terms of the COIL are in every way more permissive instead.
The AL2 has additional problems, outside of this discussion's subject matter, that are worth investigating before you adopt it for any project. While it is far from an authoritative resource, WikiVS offers some comparison in its Apache License vs COIL article.
In addition to the above, the COIL is also more generally applicable than the other licenses in the original question's attendant diagram, as the rest of them are software-specific, while the COIL refrains from tying itself specifically to software while still covering the categories of conditions applied to software by the other licenses, making combination of works covered under the terms of the COIL with other, non-software works (including documentation, artwork, et cetera -- issues of some concern to game development I would imagine) more predictably addressed by those terms.
Depending on how you interpret that diagram in relation to patent clauses in your software copyright licenses, I think COIL should fit in there either between public domain and the MIT/X11 License, or between "BSD-new" (also known as Revised BSD License or 3-Clause BSD License) and the rest of the world below and to the right of it (including AL2, MPL, and various *GPL options).
EDIT: I disagree quite a bit with Free Radical's assertions that COIL "creates legal uncertainty" (a statement made without meaningful support) and "creates license proliferation for no good reason" (there are very good reasons for the existence of the COIL, such as offering a "permissive" copyfree license with a patent clause; AL2 is not copyfree, is not nearly as permissive as some people think, and comes with some problematic clauses unrelated to the subject matter directly addressed in this discussion).
2021 EDIT: Version 1.0 of the COIL (abbreviated COIL-1.0 in the SPDX license database) has been assessed by a lawyer and fits the picture painted in these comments, except that it contradicts what Free Radical said.