Does there exist a complete implementation of the Risch algorithm?

No computer algebra system implements a complete decision process for the integration of mixed transcendental and algebraic functions.

The integral from the excellent paper of Schultz may be solved by Maple if you convert the integrand to RootOf notation (Why this is not done internally in Maple is an interesting question?)

int(convert((29*x^2+18*x-3)/(x^6+4*x^5+6*x^4-12*x^3+33*x^2-16*x)^(1/2),RootOf),x);

My experiments suggest Maple has the best implementation of the Risch-Trager-Bronstein algorithm for the integration of purely algebraic integrals in terms of elementary functions (ref: table 1, section 3 of Sam Blake, A Simple Method for Computing Some Pseudo-Elliptic Integrals in Terms of Elementary Functions, arXiv:2004.04910). However, Maple's implementation does not integrate expressions containing parameters or nested radicals (both of which has some support in AXIOM and FriCAS).

It would seem that some significant progress has been made in computing the logarithmic part of a mixed transcendental-algebraic integral by Miller [1]. Though, as far as I know, no computer algebra system has implemented his algorithm. It is also not clear if Miller's algorithm can deal with parameters, for example, the Risch-Trager-Bronstein algorithm has difficulties with the following pseudo-elliptic integral

$$\int\frac{\left(p x^2-q\right) \left(p x^2-x+q\right)dx}{x \left(p x^2+2 x+q\right) \sqrt{2 p^2x^4+2 p x^3+(4 p q+1) x^2+2 q x+2 q^2}} = - \frac{1}{\sqrt{2}}\log (x) + \frac{1}{\sqrt{2}}\log \left(\sqrt{2} y +2 p x^2+x+2q\right) - \frac{3}{\sqrt{5}}\tanh ^{-1}\left(\frac{\sqrt{5} y}{3 p x^2+3 q+x}\right),$$ where $y=\sqrt{2 p^2 x^4+2 p x^3+(4 pq+1)x^2+2 q x+2 q^2}$. My heuristic in the previously-linked paper computes this integral quickly with the substitution $u=\frac{px^2+q}{p x}$.

In regards to the mixed algebraic-transcendental case of the Risch-Trager-Bronstein algorithm, an integral which cannot be solved with Maple, Mathematica, AXIOM or FriCAS (and possibly other CAS) is

$$\int \frac{\left(\sqrt{x}+1\right) \left(e^{2x \sqrt{x}} -a\right) \sqrt{a^2+2 a x e^{2 \sqrt{x}} +cx e^{2 \sqrt{x}} +x^2 e^{4 \sqrt{x}}}}{x \sqrt{x}e^{\sqrt{x}} \left(a+x e^{2 \sqrt{x}} \right)} dx.$$

This integral is interesting as it returns two distinct messages from AXIOM and FriCAS suggesting their respective implementations are incomplete. FriCAS returns

(1) -> integrate(((-a+exp(2*x^(1/2))*x)*x^(-3/2)*(1+x^(1/2))*(a^2+2*a*exp(2*x^(1/2))*x+c*exp(2*x^(1/2))*x+exp(4*x^(1/2))*x^2)^(1/2))/(exp(x^(1/2))*(a+exp(2*x^(1/2))*x)),x)
                                                                                                        
   >> Error detected within library code:                                                               
   integrate: implementation incomplete (has polynomial part)                                                                                                                                                

While AXIOM returns

(1) -> integrate(((-a+exp(2*x^(1/2))*x)*x^(-3/2)*(1+x^(1/2))*(a^2+2*a*exp(2*x^(1/2))*x+c*exp(2*x^(1/2))*x+exp(4*x^(1/2))*x^2)^(1/2))/(exp(x^(1/2))*(a+exp(2*x^(1/2))*x)),x)
                                                                                                        
   >> Error detected within library code:
   integrate: implementation incomplete (constant residues)                                                                                                                                             

[1] Miller, B. (2012). “On the Integration of Elementary Functions: Computing the Logarithmic Part”. Thesis (Ph.D.) Texas Tech University, Dept. of Mathematics and Statistics.

Fricas, an open-source clone of Axiom, implements a considerable chunk of Risch, see http://fricas-wiki.math.uni.wroc.pl/RischImplementationStatus

Fricas is also available as a optional package of SageMath open-source system.

Edit: here how it goes in SageMath with Fricas as backend.

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sage: r=integrate((29*x^2+18*x-3)/(x^6+4*x^5+6*x^4-12*x^3+33*x^2-16*x)^(1/2),x,algorithm="fricas")                                                                                                                                     
sage: r                                                                                                                                                                                                                                
log(x^29 + 40*x^28 + 776*x^27 + 9648*x^26 + 85820*x^25 + 578480*x^24 + 3058536*x^23 + 12979632*x^22 + 45004902*x^21 + 129708992*x^20 + 317208072*x^19 + 675607056*x^18 + 1288213884*x^17 + 2238714832*x^16 + 3548250712*x^15 + 5097069328*x^14 + 6677210721*x^13 + 8106250392*x^12 + 9056612528*x^11 + 8991685504*x^10 + 7944578304*x^9 + 6614046720*x^8 + 4834279424*x^7 + 2374631424*x^6 + 916848640*x^5 + 638582784*x^4 - 279969792*x^3 - 528482304*x^2 + (x^26 + 38*x^25 + 699*x^24 + 8220*x^23 + 68953*x^22 + 436794*x^21 + 2161755*x^20 + 8550024*x^19 + 27506475*x^18 + 73265978*x^17 + 165196041*x^16 + 324386076*x^15 + 570906027*x^14 + 914354726*x^13 + 1326830817*x^12 + 1731692416*x^11 + 2055647184*x^10 + 2257532160*x^9 + 2246693120*x^8 + 1939619840*x^7 + 1494073344*x^6 + 1097859072*x^5 + 640024576*x^4 + 207618048*x^3 + 95420416*x^2 + 50331648*x - 50331648)*sqrt(x^6 + 4*x^5 + 6*x^4 - 12*x^3 + 33*x^2 - 16*x) + 150994944*x - 134217728)